(See more Quadro ramble posted contemporaneosly below!)
You may be right about The Who being the first "important group" to deliberately turn themselves into an oldies act.
I'm sure boozing and drugging and getting older have a lot to do with the loss of fire in the belly as well. The party has to end sooner or later, one way or another.
It seems to me the only realistic alternative is to do it like the Beatles or Zeppelin -- just break up already (oops! Spoke too soon on that last one. Bring on the HITS!)