Maybe McCartney, though his decline was much, much more gradual and dignified than Elvis’, and even now he is not nearly as tacky or ludicrous as Elvis became at a much younger age.
Maybe The Who as they now exist. I love the “real” Who, but,as I have said before, they really should have jacked it in when Keith died, and they definitely should have jacked it in when John died. You are the guys whose signature line was “Hope I die before I get old,” fer chrissake. You don’t actually have to die, but just retire already!
Maybe The Stones, even. Though they still rock and pull in the crowds.
Grace Slick destroyed her own considerable reputation in the '80s when she sold out with “We built this city on Rock and Roll.”
Gerry of Gerry and The Pacemakers is still performing, I think (certainly he was until last year). The joke is that it’s just Gerry and his pacemaker now. (Actually, the joke is at least 20 years old itself.)
I suppose almost anyone whose original greatness depended in large part on their unironic youthful energy and iconoclasm, but hung around in the business, without really reinventing themselves, until they got seriously old.
The unique thing about Elvis, though, was the enormous speed at which he turned from genius to a joke, even despite the abortive attempt to reinvent himself in '68. (The ‘68 show was an impressive effort, but it never really regained him anything like the popularity he had once had, and did not deserve to. The records that came out in the wake of it it were still pretty schlocky.) But perhaps it was thanks to to Elvis’ example, that the rock artists who came after him were able to avoid the pitfall he fell into for much longer than he did.
Possibly the best example of someone who went, relatively quickly, from being involved in smart, cutting edge rock to MOR schmaltz, like Elvis did, is Phil Collins. But his height was not nearly so high (nor was he the prime mover in it) nor his low nearly so low. (I was never really a Genesis fan, but I can see why people admired them.)