What happens then though, is that the replacement take a lot longer than expected, because it turns out that the complexity in the old system had more motivatio

What happens then though, is that the replacement take a lot longer than expected, because it turns out that the complexity in the old system had more motivation than expected and had a lot more problems it solved than was obvious. Fixing these issues then make the new system into almost as big of a problem as the old one, with the only difference in that now you at least have some people that understands it. For a while until enough people have left to again forget the workings of the system. Sometimes the easiest way for a company to relearn about a problem space is to rewrite it though, so it is not necessarily a bad thing to do. It can also be that the market has shifted and some of the complexity in the old system was no longer necessary, so the new system ends up easier. In any case, spending time understanding the existing system (including developers that point out corner cases and nonobvious cases) make the end result so much easier than adjusting a solution to handle a complication angle that wasn't part of the original plan.