> I now understand that a rewrite carries with it a huge amount of various risks and that people are biased to overestimate the costs of the ugly stuff they see in the old application and underestimate the risks of things they are not aware of when deciding to do the rewrite.

I see this a lot with large scale refactorings, where people usually start with the easiest case and work their way up in the complexity ladder, and by the time they reach the hairy stuff it's basically time for another refactor to account for what's been missed. I started to look at any proof of concept or refactoring proposal that doesn't take into account the complicated bits, the corner cases, with a lot of suspicion.