Additionally, as time passes you get new people working on the system that does not understand it as well as the original designers/developers, simply because they already have the system and their knowledge becomes pigeon holed inside of it. They don't have the original scope.

And then to top it off; if you did rewrite the system you not only need to reverse engineer N years of code, but also N years of bug fixes and random miscellaneous snippets that made it into the system and that no one who still works at the company understands the purpose of any longer.