When you envision how your bank will do business in five to ten years, you want to see it thriving and providing customers with a frictionless digital banking experience. You want your operations to run efficiently, have the bandwidth to launch innovative products ahead of competitors, and have teams capable of working with modern tools.
With satisfied customers who trust you because your infrastructure supports real-time payments, instant account opening, and personalized financial services. Your brick-and-mortar bank can compete effectively against digital-only banks because your technology foundation matches theirs.
The reality every bank faces today is very different, and we can't even imagine what might happen in the next decade.
The legacy technologies you invested in decades ago might be holding you back from offering the services your clients need. They are so deeply ingrained in your operations that modernizing without complete disruption seems impossible. You are definitely not alone in this, as currently, 43% of financial institutions still operate legacy systems developed over 20 years ago, according to Basikon research on core banking modernization.
There is no argument that the rip-and-replace approach of the past is out of the question. Commonwealth Bank of Australia spent over $1 billion and five years replacing legacy code. German institutions have written off entire modernization programs after years of development. The all-or-nothing approach, unfortunately, does not bring good results and requires a vast amount of money and resources that many banks simply don't have available.
But today's progressive modernization methods offer a different path. By 2026, 40% of global banks will pursue sidecar strategies, running modern cores alongside legacy systems, according to IDC projections. By 2028, that number will reach 70 to 80%. These institutions are not betting the bank on risky replacements. They are transforming incrementally, proving value at each phase, and migrating based on actual performance. This article explores the progressive modernization playbook that works.
Why Big Bang Core Replacement Projects Fail
Currently, 43% of financial institutions still operate legacy systems developed over 20 years ago while their maintenance costs for legacy systems consume up to 70% of IT budgets.
Among banks with fewer than two years remaining on their core provider contracts, 60% report dissatisfaction and 40% are actively considering platform switches, according to the American Bankers Association's 2024 Core Platforms Survey. Yet when renewal arrives, only 20% make the switch, underscoring how problematic digital modernization can be, especially when it's done within highly regulated systems.
This gap between intent and action reveals the real issue: fear of catastrophic data loss.
What Are the Three Fatal Flaws of Big Bang Migration
- 1. Problematic Risk Profile: One failed cutover can take down the entire bank. Commonwealth Bank's $1 billion price tag and five-year timeline represent a level of risk few institutions can afford today.
- 2. Timeline Assumptions Are Wrong: McKinsey research shows core replacement projects routinely run over budget and take years to complete, often exceeding initial estimates by 50% or more.
- 3. The Talent Gap Is Real: Few internal teams have experience managing large-scale migrations. Meanwhile, 90% of US banking core software is considered legacy, according to TechMagic industry analysis. Finding COBOL experts who can bridge to modern architectures has become increasingly difficult.
More than half of mid-market banks with assets between $10 billion and $100 billion now favor progressive transformation to gradually reduce legacy dependence. Forrester Research identified banks globally becoming less risk-averse and more receptive to fintech partnerships through decoupled approaches. The solution is choosing progressive deployment over high-risk, all-or-nothing approaches.
1. Understanding the True Cost of Staying on Legacy Systems
Complexities and risks aside, you might be postponing the modernization journey for another reason: the perceived cost. What you may not realize is that you are already paying a steep price.
The $93.6 Million Question
FIS and Oxford Economics research quantifies the cost of legacy systems: $93.6 million in annual losses per institution. The breakdown reveals where your money disappears. Operational inefficiencies cost $11.2 million yearly. Payment and processing friction costs $5.6 million. Human error and rework cost another $5.6 million. Add nearly $60 million more for cyberthreats, fraud, and compliance issues, and the case for modernization becomes undeniable.
These losses compound year after year. A five-year delay in modernization means $468 million in cumulative losses that could have funded transformation multiple times over.
The Maintenance Burden That Consumes Your Budget
Beyond direct losses, legacy systems drain resources through maintenance. Research from Basikon and TechMagic shows these aging platforms consume 70 to 78% of IT budgets on maintenance alone. That leaves only 20 to 30% for innovation, new product development, and competitive initiatives.
McKinsey research reveals an even starker reality. Operating costs for banks still running outdated cores average 10 times higher than those with next-generation systems. Every dollar spent maintaining legacy infrastructure is a dollar not invested in growth, customer experience, or market expansion.
Currently, 90% of US banking core software is considered legacy, according to TechMagic industry analysis. The pool of COBOL and mainframe experts continues to shrink as talent retires or moves to other roles. Finding specialists who understand decades-old systems has become increasingly difficult and expensive.
The Competitive Gap is Widening Daily
While you maintain legacy systems, competitors build advantages. According to The Financial Brand's 2026 Retail Banking Trends and Priorities Report, 62% of banks planned to offer real-time payments in 2026, up from 49% in 2024. Yet many traditional banks lag behind fintechs and neobanks that built on modern infrastructure from day one.
Paul van der Merwe, head of enterprise architecture at Standard Bank Group of South Africa, notes that innovations in core banking, payments, and compliance are now expected to handle real-time demands by nature. Legacy systems were not designed for this reality. Every day you delay modernization, the gap between customer expectations and your capabilities widens.
The Risk of Staying Put
Among banks with fewer than two years remaining on their core provider contracts, 60% report dissatisfaction and 40% are actively considering platform switches, according to the American Bankers Association's 2024 Core Platforms Survey. Yet when renewal arrives, only 20% actually make the switch.
This gap between intent and action reveals the fundamental problem: fear of catastrophic failure during migration. The perceived risk of modernization feels greater than the known pain of legacy systems. However, research shows that staying on legacy systems increases operational risk by up to 30% compared to modern platforms.
Forrester Research predicts that cloud-related security breaches will increase by 50% in the next five years. Legacy systems with outdated security architectures become increasingly vulnerable. The risk of not modernizing now exceeds the risk of transformation.
What These Costs Mean for Your Future
The combined impact of direct losses, maintenance burden, competitive disadvantage, and increasing risk creates an unsustainable trajectory. Yet all these stumbling blocks could be addressed by embracing progressive modernization strategies that leverage modern technologies without betting the bank on risky replacements.
The question is not whether to modernize. The question is how to modernize without the catastrophic risks that have plagued big bang approaches. That is where the sidecar strategy comes in.
2. The Progressive Modernization Playbook: Why 40% Choose Sidecars
The banking industry has learned from costly failures. Commonwealth Bank's billion-dollar journey, German banks' abandoned programs, and countless stalled transformations have taught a clear lesson: big bang replacement rarely works. The industry has responded decisively.
What Is the Sidecar Strategy?
A sidecar core is a separate modern banking system that runs parallel to your legacy infrastructure. Unlike full replacement, it handles only specific customer segments, products, or services—typically starting with 1 to 5% of your customer base. While your legacy core continues managing bulk operations, the sidecar proves new capabilities with a contained subset of your business.
This approach is not theoretical. IDC projects that 40% of global banks will pursue sidecar strategies by 2026, rising to 70 to 80% by 2028 as banks gain experience with hybrid approaches. Progressive modernization is rapidly becoming the industry standard.
Why the Sidecar Approach Works
According to Forrester research on banking architecture, digital-only banks serve as proving grounds for banking innovation precisely because they are built natively on modern architectures without legacy burdens. The sidecar strategy allows traditional banks to replicate these advantages without abandoning existing infrastructure.
More than half of mid-market banks with assets between $10 billion and $100 billion now favor progressive transformation to gradually reduce legacy dependence. Banks that were paralyzed by fear of big bang failure now see a viable path forward.
Real-World Applications
Instant Payments: Your customers increasingly expect real-time payment capabilities. Banks are deploying sidecars specifically to handle instant payment users, migrating FedNow or RTP customers to modern cores without touching legacy settlement systems.
Digital Brands: Goldman Sachs demonstrated this with Marcus, launching a digital-only brand on modern infrastructure while maintaining existing operations on legacy systems. Traditional banks can follow this playbook, targeting Gen Z customers, small business banking, or geographic expansion.
New Products: Seasonal offerings and limited-time products provide ideal proving grounds. You test new product-market fit on flexible platforms before committing to full integration.
Technology, Processes, and People: The Holistic View
As you embark on sidecar modernization, it quickly becomes clear that this is not strictly about technology. It is as much about the processes that drive every interaction throughout your bank.
Your sidecar runs on cloud-native architecture with API-first design. McKinsey research shows 44% of banks expect to decrease costs by more than 10% through API efforts, while 31% expect to increase revenues by more than 10%. The API-enabled banking market is projected to generate over $25 billion in annual revenue globally by the end of 2026.
Running dual systems requires new operational processes. Your customer service teams need access to both platforms. Your reporting must consolidate data from multiple sources so these challenges force process improvements that benefit your entire organization.
The people challenge is equally critical. Research shows 78% of banking leaders are prioritizing upskilling, with the global industry expected to spend $10 billion annually on upskilling initiatives by 2024. Progressive modernization helps because you are not asking your entire organization to transform overnight. You build expertise gradually.
Industry data shows banks investing in reskilling report 33% higher employee retention rates. The investment pays back within 12 to 18 months through reduced turnover alone.
Forrester Research identified banks globally becoming less risk-averse and more receptive to fintech partnerships through decoupled approaches. You do not need to choose between stability and innovation. The sidecar strategy delivers both.
3. Building the Foundation: Infrastructure, APIs, and Data Strategy
Successful sidecar modernization requires building the right foundation before migrating workloads. This foundation spans three critical areas: API infrastructure that enables dual-core operations, strategic data migration that reduces costs, and talent development that ensures your team can manage modern platforms.
The API Abstraction Layer: Your Translation Bridge
An API abstraction layer acts as middleware between your legacy cores and modern applications. It decouples your internal systems from the underlying core banking platform, enabling innovation without core dependency. More importantly, it makes dual-core operations possible by providing a single interface that works with both systems simultaneously.
Oliver Wyman's research on core banking modernization documents a proven three-layer architecture pattern:
- Layer 1: Core Abstraction API – Provides standardized interfaces for all banking operations. Your internal applications call this layer, never directly touching the cores beneath.
- Layer 2: Core Banking System Connect – Handles the complexity of communicating with legacy systems. Manages protocol translation, connection pooling, and error handling to keep the abstraction API above clean.
- Layer 3: ETL Data Synchronization – Ensures customer account balances, transaction histories, and reference data remain consistent across both systems during dual-core operations.
Real Implementation: Proving It Works
One American bank implemented this approach during its core modernization. The bank had operated a 30-year-old core with poor data access, scalability issues, and slow response times. Rather than attempting direct replacement, the implementation team built wrapper APIs that segregated internal systems from the external core.
This layer allowed internal applications to scale without dependency on the core. The result: the new core could be connected through the API abstraction layer with minimal disruption to existing operations, and the bank gained flexibility to adapt to future changes without another painful migration.
The abstraction layer also enables AI integration. According to Forrester, when banks pair generative AI with process mining, they can automate process reconciliation, exception handling, and regulatory reporting. These capabilities require the flexible, API-driven architecture that abstraction layers provide.
Strategic Data Migration: Not Everything at Once
Data migration breaks more modernization projects than any other factor. The solution is tiered migration, not wholesale movement. BAI research shows most archived data experiences minimal access after the initial 18 months, suggesting not everything needs immediate migration to the new core.
Structure your data in three tiers:
- Tier 1: Active Migration (0 to 18 months) – Recent transactions, active accounts, and current product holdings require full migration with real-time synchronization. This is your most expensive tier.
- Tier 2: Accessible Archive (18 months to 7 years) – Historical records become searchable browser-based archives with no conversion required. You maintain compliance without migration costs.
- Tier 3: Cold Storage (7+ years) – Regulatory retention data stays on tape or object storage, retrieved only for audits or litigation.
This tiered strategy reduces migration costs by 30 to 50% while maintaining compliance and accessibility. You focus expensive migration resources only on data that requires modern core functionality.
Modernize Peripheral Systems Before the Core
A critical but often overlooked step is to modernize peripheral systems before migrating the core. Update integration layers, middleware, and customer-facing applications to communicate through APIs rather than direct core connections.
The abstraction layer enables you to update systems incrementally without waiting for core replacement. Banks following this approach report faster overall transformation timelines because peripheral modernization can happen in parallel with core planning, not sequentially after it.
Invest in Your People, Not Just Technology![5_image.jpg]()
The older your technology, the greater the leap from legacy to modern. The technical talent onboarded decades ago has retired or moved into new roles, while developers entering the industry lack understanding of the deeply embedded, outdated code in your systems.
Oliver Wyman research shows that program transformations often fail due to gaps in required skill sets and reliance on non-permanent staff. Banks should upskill existing staff to manage new platforms while working with external partners for knowledge transfer.
Structure your talent program across three tracks:
- Technical upskilling: Cloud-native architecture, API design, modern programming languages, and DevOps practices. Research shows 82% of banking executives see AI and machine learning as critical skills for the future workforce.
- Operational readiness: Use simulation-based training on new platforms. Allow employees to practice workflows before go-live. This approach shortens time-to-proficiency by weeks and identifies process gaps before deployment.
- Change management: Explain why change is happening, how roles evolve, and what support systems are available and start training six months before platform deployment.
Section 5: A Phased Execution Model
Once your sidecar proves value with the initial 1 to 5% of customers, the question becomes: how do you scale without introducing the risks you initially avoided? Here's a model that we've seen works, as it provides phased execution where you can be in control of the process.
The Six-Phase Scaling Model
Phase 1: Initial Deployment (Months 1-6)
Deploy the sidecar with a controlled customer segment—new account openings, a specific branch, or a digital-only product line. Focus on stability and learning rather than volume. Your goal is proving the platform works in production.
Phase 2: Validation and Optimization (Months 6-12)
Analyze performance data. Identify bottlenecks. Refine processes based on actual usage patterns. This phase determines whether you proceed or pivot. IDC research shows this validation period is critical—rushing past it accounts for 60% of scaling failures.
Phase 3: Controlled Expansion (Months 12-18)
Expand to 10 to 15% of your customer base. Target specific segments where modern capabilities deliver clear advantages—small business banking, instant payment users, or mobile-first customers. Maintain the discipline of measuring before scaling further.
Phase 4: Accelerated Migration (Year 2-3)
With confidence established, accelerate migration to 30 to 50% of customers. This phase moves fastest because your team has learned, processes are refined, and stakeholder confidence is high. According to Oliver Wyman research, banks in this phase typically migrate 10 to 15% of customers quarterly.
Phase 5: Majority Migration (Year 3-4)
Migrate 70 to 80% of your customer base. Your legacy core now handles only edge cases, complex product configurations, or customers requiring specialized servicing. The balance of power has shifted—the modern core is primary, legacy is secondary.
Phase 6: Legacy Sunset (Year 4-5)
Plan the controlled retirement of your legacy system. Migrate remaining customers, archive historical data according to your tiered strategy, and decommission outdated infrastructure. This final phase requires as much planning as the initial deployment.
Managing the Migration Without Customer Disruption
Customer communication is critical at every phase. Communicate benefits clearly: faster transactions, better mobile experience, new capabilities. Be transparent about what changes and what stays the same and give customers advance notice and support channels.
Offer opt-in opportunities before mandatory migration. Early adopters provide valuable feedback and become advocates so when mandatory migration begins, do it in waves with robust support coverage. BBVA's progressive transformation provides a template. The bank modernized without major customer disruption by focusing on clear communication, strong support, and demonstrable improvements to user experience.
When to Accelerate vs. When to Pause
Your phase-gate KPIs tell you when to accelerate and when to pause. If customer satisfaction on the modern core exceeds legacy benchmarks and operational metrics are strong, accelerate. The beauty of progressive modernization is the optionality it provides. You're never locked into a timeline that market conditions or internal challenges make unrealistic.
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The Path Forward: Start Your Progressive Modernization Journey
The industry trajectory is clear. By 2026, 40% of global banks will pursue sidecar strategies, rising to 70 to 80% by 2028. Progressive modernization is becoming the industry standard because it provides optimal risk-adjusted returns while enabling competitive capability development.
Over 85% of financial institutions plan to invest heavily in transforming their core banking systems by 2025. Banks spend 78% of IT budgets maintaining legacy cores. Operating costs for outdated systems run 10 times higher than next-generation platforms. And 55% of banks say legacy core limitations are the most significant barrier to achieving business goals.
Conclusion: The Path Forward
The banking industry stands at an inflection point as legacy systems that powered decades of growth have become barriers to the future. The $93.6 million annual cost of inaction compounds while competitors pull ahead, but there's a solution in sight, and that's a progressive modernization approach.
The sidecar strategy, which suggests running modern cores alongside legacy systems, can help deliver transformation without incurring existential risk to the bank, its data, or IT budgets. By 2028, 60 to 70% of banks worldwide will start pursuing some type of digital transformation, and the question is not whether to adopt progressive modernization, but how quickly you can begin.
The banks winning this transformation are not the ones with the biggest budgets or the most aggressive timelines. They are the ones with the discipline to prove value incrementally, the patience to learn from each phase, and the strategic vision to see beyond quarterly results to the institution they're becoming.
Every digital modernization journey begins with a single step: code audit, consultation with a software development provider that understands your situation, and diligent planning of the process.
Contact our team today to explore how our expertise in progressive core banking modernization can help you navigate the transition from legacy to modern systems using proven sidecar strategies.
