I don't believe any band can over stay their welcome so long as people are still buying tix and going to the shows. Now, if you were once an arena attraction and are now playing 150 capacity pubs, you've probably over stayed your welcome. Every year someone like The Who or The Stones tour, and then someone runs their mouth about these bands staying around too long yadda, yadda, yadda. Ya know what - if you don't like them, don't buy a fucking ticket, there are plenty of people who will $100 to see these bands play arenas, ampitheatres, and stadiums. On another note, I really hate that whole "hair" genre. But why would I want Poison et al to go away? If people want to go see them every summer, more power to them.
Can a band "tarnish their legacy," by touring after their prime and putting out lessor singles and albums? Sure, but does a new song really diminish how good an old song was???? The answer to that has to be no. So once again, if you don't like the new Stones or new Aerosmith (I don't dig either bands newer material) don't listen. A new Aerosmith album that sucks, doesn't diminish the fact the Toys in the Attic is epic. Rant over.