Recently, I had to work on an old C++ builder 4 (1998) application. The IDE couldn’t be installed anymore on Windows 7, there were global variables and singletons everywhere, client-based compilation directives, no separation between UI, business logic and data layer, no class isolation, memory leaks all over the place, compiler failure as soon as the source code got too big, dependencies on obsolete COM objects, etc. A nightmare. It looked like the perfect example of an application that should be rewritten from scratch, and it made me wonder when is it a good time to completely rewrite a product ?
Short answer : Never.