My recent experience is that a lot of the system was not as good as it could have been because it's a hard problem. The company got bought out and all the new bosses come in and say all the existing code and way things are done is crap. Then, a year later, with everyone sacked and new people brought in, once again, these guys say the same thing and promise they are the top guns to fix it all up. I just checked out on the third round.

Ah, that's definitely true. But that feels like an orthogonal concern to you being an existing dev working on legacy stuff vs greenfield stuff... unless the new boss likes the greenfield stuff for some reason. What you are talking about is more of an ever-present possibility of "new management fallout" in almost any field.