The verdict in three sentences
Keeping a legacy application (VB6, obsolete PHP) costs a growing stream of technical debt each year, plus the risk of a blocking outage. A custom rebuild costs more upfront but delivered in 20 to 28 weeks it removes the risk and restores security, performance and extensibility. The strangler strategy (progressive module-by-module migration) cuts the risk of a big bang and smooths the investment, with payback typically reached in under 3 years.
Keep or rebuild: the decision calculation
Legacy is not free just because it is written off: each fix costs more and the risk grows. Here is the 3-year decision comparison (day rate EUR 650 to 900):
| Criterion | Keep the legacy | Custom rebuild |
|---|---|---|
| Direct 3-year cost | ~EUR 110,000 (growing debt) | EUR 150,000 to 260,000 (one-off) |
| Blocking-outage risk | High and rising | Low |
| Security / compliance | Obsolete, vulnerabilities | Up to date, encryption |
| Performance | Degraded | +50 to 300% |
| Extensibility | Near zero | Modular, API |
| Reliance on 1 rare dev | Critical | Eliminated |
Legacy looks cheaper year by year, but technical debt accelerates: maintenance often climbs from EUR 25,000 to EUR 45,000 a year over 3 years, while never solving the underlying risk.
Strangler vs big bang: two migration strategies
The strategy choice drives both risk and cash flow. Here are the two approaches costed:
| Strategy | Principle | Cost | Timeline | Risk |
|---|---|---|---|---|
| Big bang | Replace everything at once, single switch | EUR 150,000 | 20 wks | High (tunnel effect) |
| Progressive strangler | Module by module, temporary coexistence | EUR 180,000 to 260,000 | 24 to 28 wks | Low (continuous delivery) |
| Audit + data migration | Mapping, extraction, cleaning, migration | EUR 24,000 to 42,000 | 3 to 5 wks | Prerequisite for both |
The strangler costs a bit more but avoids the tunnel effect: each migrated module goes live immediately while the old system runs in parallel until retired. It is the recommended choice for a critical business application in active use.
Mini case study
Aoife, IT director of a distribution SME in Dublin. Her VB6 management software crashes twice a quarter and now hangs on a single external developer who charges a premium. Maintenance costs her EUR 30,000 a year and rising. She opts for a strangler rebuild at EUR 215,000 over 26 weeks, migrating 12 years of data. Result: outages gone, security restored, and productivity that saves about EUR 68,000 a year in staff time and avoided errors. Combined with the removed maintenance cost, payback is reached in 30 months.
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FAQ
How long does a full legacy rebuild take?
Expect 20 to 28 weeks depending on strategy. The strangler takes a bit longer but puts modules in production from the first weeks.
Can we migrate 10 years of historical data?
Yes. Audit, extraction, cleaning and migration cost EUR 24,000 to 42,000 depending on source-data quality and schema complexity.
Is the strangler pattern really less risky?
Yes: by migrating module by module with coexistence, you avoid the big bang's single switch. Each delivery is testable, sharply cutting the risk of an operational shutdown.
What is the real cost of doing nothing?
Beyond the annual maintenance, a blocking outage can cost several days of activity, and dependence on a rare developer becomes critical.
Will the custom build perform better?
Typically +50 to 300% on processing times, with a modular architecture and APIs that finally let you evolve without breaking everything.
Let's scope your project. Share your current software, its age and the data volume to migrate: we will scope an audit and a migration 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.
