Why should he quit? Shoot, from what I've seen, he'd doing the same stuff as always, just with new twists.
He's always supported acts that obviously were generations separate from him. lol I remember once he had Billy Idol on his show...who appeared to be in a drugged out coma...Dave made the most of it, saying, "we probably wouldn't be the type of guys who'd hang out together, right?"
I do miss him having Cher or Shirley MacLain on his show who ended up calling him an "asshole" and walking off stage. lol
I can't understand why he should quit,,,and in so far as who could replace him, it is late at night after all, and with the recent switch over to digital broadcast and band splitting making three channels possible on just one channels bandwidth, I cant see why anyone who could replace him shouldn't have their own late night show, too, whole bunches of late night shows.
I go to bed about 9:30.