It depends. One system I worked on was an information system in VB6 which was being used as a base platform to build domain-specific ERP implementations. Deplo

It depends. One system I worked on was an information system in VB6 which was being used as a base platform to build domain-specific ERP implementations. Deployment in their environment to anything non-browser-based was a nightmare. There was overlap between domains although there was shared data (customer and supplier information, product database, that kind of thing). The obvious thing to do here was to let each domain (i.e. team) build their own solutions in the most effective platform for their domain whilst integrating via defined interfaces. That's not what happened; when I left few teams had anything working and those who did spent the majority of their time on deployment issues. Another system I worked on was initially built by a very junior team under tight time and budget pressure under leader who believed in never saying "no" to the customer. They created a horrible mess (most of the code was in a couple of files). The system was deployed by two/three customers. Testing was done during deployment; the first version worked well enough to go into production. A year later everything had ground to a halt; there was no formalised testing and the developers couldn't fix one bug without introducing several more. One very smart developer started to "rewrite from the inside" by implementing new features using clean and modern techniques. At that point I took over the team. We doubled down on this "rewrite" whilst adding manual tests (and later automated tests) to cover the most important flows. We only rewrote features when they needed to change and in some cases removed functionality when particularly hairy features no longer worked. Otherwise I advise against rewrites; if the system was reasonably built and the platform viable then rewriting is just a waste of money.