The verdict in three sentences
Modernizing an obsolete internal system costs 50,000 to 200,000 EUR in 2026, but the real number to compare is the cost of inaction: blocking outages, departure of the original developer, inability to evolve. The safest strategy is rarely a full rewrite (big bang): the strangler pattern, replacing the old system piece by piece, cuts the risk. The decision is made on 3-year TCO, not on build cost alone.
The three modernization strategies
You do not rebuild a legacy system like a greenfield project: it is in production and must not stop. Three strategies dominate in 2026.
| Strategy | Principle | 2026 cost | Risk | Duration |
|---|---|---|---|---|
| Full rewrite (big bang) | Rebuild all, single cutover | 120,000 to 200,000+ EUR | High | 8 to 14 months |
| Strangler pattern | Replace module by module | 80,000 to 160,000 EUR | Medium | 6 to 12 months |
| API encapsulation | Keep the core, expose APIs | 50,000 to 90,000 EUR | Low | 3 to 6 months |
Encapsulation is cheapest but does not solve the underlying debt; the strangler offers the best risk/value balance for most mid-market firms.
The measured cost of inaction
Keeping an unmaintained system is not free: it is a risk that materializes sooner or later. Here are the hidden cost items of the status quo.
| Status-quo risk | Probability / year | Estimated impact |
|---|---|---|
| Blocking outage without support | Medium | 5,000 to 30,000 EUR / incident |
| Departure of original developer | High over 3 years | Loss of critical knowledge |
| Unpatched security flaw | Medium to high | Data breach, GDPR |
| Inability to add a feature | Certain | Commercial opportunity cost |
| Artisanal maintenance overhead | Continuous | +30 to +50 % vs modern stack |
Over three years, the accumulation of these costs frequently exceeds the budget of a gradual modernization.
Mini case study
Nadia, IT director of an industrial mid-market firm in Amsterdam, depends on a 15-year-old in-house ERP maintained by a single developer near retirement. A recent outage cost 22,000 EUR (1.5-day production stop).
She costs it out: big-bang rewrite at 170,000 EUR (14 months, high risk) versus strangler pattern at 120,000 EUR spread over 10 months, migrating the billing module first (the most critical). By choosing the strangler, she first secures the breaking point, transfers knowledge before the original dev leaves, and spreads the budget over two fiscal years. Estimated 3-year TCO about 35,000 EUR lower than big bang, with production risk halved.
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FAQ
Should I rewrite everything at once?
Rarely. Big bang concentrates all risk on a single cutover. The strangler pattern, module by module, lets you validate at each step and keep the system in production.
How much does modernization cost in 2026?
From 50,000 EUR (API encapsulation) to 200,000 EUR and up (full rewrite of a complex ERP). The budget depends mostly on the volume of business rules to carry over.
What is the risk of doing nothing?
Unsupported outages, loss of the original developer, unpatched security flaws and inability to evolve. These hidden costs often exceed the rebuild budget over 3 years.
How do I avoid losing the old system's business rules?
Document and re-test each rule during migration, ideally before the original developer leaves. The strangler pattern eases this gradual transfer.
Can I keep the current software a few more years?
Yes via API encapsulation (3 to 6 months, 50-90k EUR), which extends the core's life while modernizing interfaces. It is a transition, not a permanent fix.
Let's scope your project. Describe your current software (age, stack, criticality) and your continuity constraints, and we will propose a costed modernization strategy. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
