I saw both Tull and the Damned in Austin a couple of years ago, and a good buddy of mine saw the Who in Dallas in 2000. He said they were terrific, simply because Townshend was so into it. Ditto for me for the Tull and Damned shows, the former because Ian Anderson looked like he was having a blast and the Damned because they were promoting a great new album (Grave Disorder) and played with all the enthusiasm of a new band.
Point being, if the bands in question are still playing their music because they love it and wouldn't have it any other way to play "I Can't Explain" or "Aqualung" or "Smash It Up" for a living, then yes, I think they should go on performing until they collapse. It's the bands that reunite for no other reason than to score a few bucks, or because their creative wells run dry and they can't do anything else but recycle the old hits, that should hang it up.
Or the ones that try to get by without significant members, a la the Doors with Ian Astbury in Jim Morrison's place, Lynyrd Skynryd without Ronnie Van Zant (indeed, with ex-members of Blackfoot and the Outlaws in the band, they're more of a Southern rock Frankenstein's monster playing Skynryd covers than a real band) or Thin Lizzy without Phil Lynott.
Of course, one could make the same argument about the upcoming DTK/MC5 tour without the late Fred Sonic Smith and Rob Tyner, but at least the survivors are billing it as more of a celebration of the MC5 than a band reunion. And why shouldn't they play those songs? They helped create them.
I think ultimately you have to look at such a question ("Any views on whether these bands should still be performing nearly four decades after their debuts?") on a case-by-case basis.
