I've worked on some of those "old, large, working C/C++ codebases" and the ones that are not clean are the ones where you are terrified of making a small change to fix a bug, especially close to release, because you have absolutely no idea what might break. Two copies of the same code? Great, which one do I fix? Both? How do I test that it worked, I can't even figure out where #2 is activated in the UI?! Is there something a little different about them that I need to watch out for? And test cases for said codebase? Hahahahaha!
