For clarity, when I say IT Architect, I’m referring to professionals responsible for the design, integration, and strategic alignment of technology systems across the enterprise - spanning infrastructure, applications, data, and security.
Legacy systems are the silent handbrakes holding many organisations back. Yet they continue to underpin some of the most critical business operations - from financial services and healthcare, to government and infrastructure.
For IT Architects, the challenge is clear. How do you modernise core systems without destabilising the business?
Why Legacy still dominates in 2025
Despite the explosion of cloud-native platforms and SaaS tooling, legacy estates still account for a significant percentage of enterprise IT spend. Mainframes, COBOL systems, monolithic applications, and tightly coupled integrations still drive critical workloads.
Why?
Because they’re:
- Deeply embedded in operations
- Often highly customised over decades
- Tied to regulatory, data, or risk constraints
- Expensive (and dangerous) to replace without clear business sponsorship
But ‘legacy’ doesn’t have to mean ‘left behind.’
It simply means your architecture needs a modernisation strategy that’s built for reality, not perfection.
The Architect’s role - from Firefighter to Strategic Navigator
IT Architects have a unique seat at the table - able to see both the operational reality and the strategic horizon. But many are still reacting to short-term pain: integration blockers, unsupported technology, brittle codebases.
Modernisation requires a shift from reactive to intentional.
Here’s how:
1. Assess, don’t Assume
Before any roadmap is drawn, you need a maturity-led baseline:
- What systems are truly business-critical?
- Where are the dependencies and integrations?
- What’s the risk profile of each application (security, compliance, support)?
- What’s the cost to run vs cost to modernise?
Tools like Application Portfolio Management (APM) and Business Capability Mapping can uncover misalignments, hidden duplication, and technical debt.
Quick win: Start with a heatmap that scores apps across value, complexity, and cost.
2. Decouple to De-risk
One of the most effective approaches is progressive decoupling - extracting business logic and services incrementally from monolithic systems and exposing them via APIs or microservices.
- Build a strangler fig pattern (replace components over time)
- Implement data abstraction to separate data access from core logic
- Use event-driven architecture to reduce tight coupling
This allows teams to iterate and deploy modern functionality without needing to rewrite everything.
3. Bring Business and IT together early
One of the biggest modernisation killers?
Lack of business engagement.
Enterprise Architects must translate technical opportunities into business outcomes:
- Increased agility to respond to market changes
- Reduced time to onboard customers or partners
- Improved resilience and risk posture
- Stronger data quality and decision-making
Modernisation is not an IT project. It’s a business enabler. And your architecture roadmap must tell that story.
4. Use Cloud strategically (not emotionally)
Cloud is not a strategy .. it’s a capability.
Lift-and-shift isn’t always cheaper or smarter. The right move might be:
- Re-platforming to PaaS to modernise operations
- Refactoring specific high-value services
- Retiring unused or low-value legacy workloads entirely
Hybrid architectures are the norm, not the exception. Build for interoperability, not just lift-and-replace.
5. Security and Compliance - embed from the start
Legacy systems often create blind spots in:
- Identity & access control
- Patch management
- Data governance
Use modernisation as a trigger to embed zero trust, automate compliance checks, and integrate with central IAM platforms.
If your architecture doesn’t simplify and secure the estate, you’re only solving half the problem.
6. Define what ‘modern’ means for your Organisation
Modern doesn’t have to mean microservices and containers.
It could mean:
- Supportable by current skills and tools
- Scalable to meet future demand
- Modular and loosely coupled
- Aligned to capability-driven investment decisions
The point is: make it measurable. And evolve it as your business evolves.
Modernisation is a Capability, not a Project
The most successful IT Architects we work with are building repeatable, value-led transformation frameworks that:
- Avoid disruption
- Engage the business
- Prioritise outcomes over activity
Entasis Partners help Clients strategically reshape legacy estates through architectural leadership, cross-functional delivery, and outcome-focused teams. Whether you're designing your first roadmap or deep into phased delivery, we can help build the right foundations for scalable, sustainable change.
