So you don't think there is a problem in the code base, but you say: " legacy code is so high and you have to keep compatibility with lots of existing user code". That's a contradiction.
I mean that the code works perfectly in the sense that the behaviour conforms to the specification, but it is hard to modify it. So, there's a problem with the code base, just one that is not visible if you don't want to extend the program (one can argue that it's the worst kind of problem).
A lot of people have thought about rewriting (parts) of TeX, but the amount of legacy is so high and you have to keep compatibility with lots of existing user code.