In my experience, when you have a system that works, is creaky, and you have to spend days to understand how thing work, there is at best a significant gap between actual and specified behavior. Most of the times, there is actually no spec at all, it is mostly implementation defined. And you rarely have the same team as the one who did the previous team (people left, have to work on other projects, etc...).
That's one of the fundamental issue with complete rewritea: the systems you could most easily rewrite are often the one you don't need to.
That's one of the fundamental issue with complete rewritea: the systems you could most easily rewrite are often the one you don't need to.