There are a variety of reason that a migration of a legacy system may be needed:
Legacy languages are hard to support. The legacy languages and development tools needed to support the legacy system are increasingly difficult or expensive to obtain.
Visual FoxPro is a good example. VFP was one of the most powerful languages I have ever known and although it still works, Microsoft unfortunately ended support and one day not too distant the legacy code will stop to work. For those who love Visual FoxPro as I do it's possible to rewrite the software using PHP and preserving the layout.
People are scarce. People that know the legacy languages and technologies are becoming difficult to find and retain. Younger staff are reluctant to learn legacy systems because it does not appear to advance their long-term career.
The underlying platform is hard to support. Many legacy systems run on legacy hardware systems. Such hardware systems are becoming more expensive to maintain, and personnel that know these systems are also more difficult to find.
Legacy software does not integrate well with other IT systems. The architecture of legacy languages often does not lend itself to building bridges to other IT systems that have grown up around it.
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