The inherent problem with comics is that because they're made to just go on forever, anything that can be done can just be undone.  We've seen classic characters age, die, or otherwise be put on the shelf before, with the attempt to get new characters the spotlight, but eventually someone will want to dust off that old concept and bring the original back.  If someone writes the single greatest possible way to end the Batman/Joker rivalry, in a way that leaves absolutely no loose ends and no possibility for a rematch, well, it doesn't actually matter because somewhere down the line, someone will bring it back.  Peter Parker has been a young man for over half a century; Bruce and Clark have been about 40-ish for 75+ years; the idea that someone can just now put some gray hairs on them and expect audiences to buy their replacements just isn't going to stick.

Because superheroes-- the big 'iconic' ones in particular-- are more or less eternal, they've become the collective property of the popular consciousness rather than any one particular writer and artist, and with that comes a status quo and a set of expectations that have to be met.  Most of the time, radical departures, reinterpretations, or shake-ups to the lore are met with controversy at best, if not outright rejection.  You can't get away with doing to Superman what Morrison did with Animal Man, f'rinstance.  I will say that adding new characters is usually well-received, but taking them away or making them clash with the common perception of them is a gamble that rarely ever pays off.

In that respect, I kinda wish comics would get away from having a strict ongoing continuity and just let creators have free reign in self-contained standalone stories.  That way, they can take more risks and experiment more without as harsh consequences from the general public because hey, if you don't like that story, it doesn't overwrite the ones you do like.  And it's not like we don't already have proof of concept in that regard-- most of the best-regarded superhero books of the past twenty or thirty years have been stories that happen outside of the monthly canon anyway.