in general, the list of bands that should have quit in embarrassment is pretty long, but there are also quite a few that have aged like fine wine.
Propagandhi - band is 30 years old, still releasing albums, and every one is better than their last.
Weird Al Yankovic - over 40 years of performing, and he's still relevant, still talented, still hilarious, and still one of the best live shows in rock.
Ramones - toured for 30 years and while you can argue about the quality of their albums (i think they were still pretty good at the end) their live shows never lost a step.
Buzzcocks - 40 years and they are still fantastic live, and (occasionally) releasing albums that don't suck.
Daycare Swindlers - they're a local group that has been around for about 20 years and they still amaze and inspire me. if they died, i would die a little.
Fugazi - hung around about 17 years, and was still producing urgently essential music up until the minute they disbanded. Ian Mackaye is a dude whose picture should be in the dictionary next to the phrase "artistic integrity". His bands have never felt like they did anything thing for the purpose of cashing in or rehashing the past. His bands were always evolving, and always knew when to quit.