Time marches on and performers age. Some age more gracefully than others, and in time the musical abilities of some of these iconic performers diminish. It's a natural part of life. I get the sense that some critics and writers don't necessarily take this into account when they review a performance. And the tone of music criticism seems to have become more harsh and sensational as various music media outlets try to survive.
I got to see B.B.King at the 2010 Crossroads Festival. He really didn't play his own set, per se. Rather, he sat in a semi-circle with select players and led them through about 20 - 30 minutes. He was joined by Eric Clapton, Jimmy Vaughn, Robert Cray and Cray's band. B.B. sang, told stories and talked with the crowd. He traded solos with everyone else on stage, but his playing was clearly not up to the level of Clapton or Cray. I think B.B. recognizes his status as this elder statesman, and the audience is genuinely happy to see him. But he is clearly NOT the B.B. King of 20 or more years ago. Still, the crowd loved him that night.
Contrast that performance with another that I'd seen earlier in the day. That was Johnny Winter playing a half dozen songs with some serious backing players that included Warren Haynes and Derek Trucks of the Allman Brothers and Cesar Rojas and David Hidalgo of Los Lobos. All of those guys can play! But that set was very awkward. In that instance Johnny Winter's playing was abyssmal. I had never seen him perform, but I honestly found it to be rather sad. Johnny Winter is a legendary player, but his performance was so poor that it wasn't included in any of the live recordings of that show. And the whole time he was playing, I felt genuine pity for him. But that turned to anger when it became clear that Johnny Winter was content to take all of the soloing time during "Red House", spurning all of the other guitarists on stage.
As for anyone telling these guys to call it a day? One would hope that these guys are self-aware enough to make that call. But then there are the other factors. What if they do retire? Will they lose the one thing in their lives that gives them purpose, which is performing? Are they comfortably set up for retirement? Do they have business committments that are forcing them to keep performing? I think that all of these are serious questions that have to be taken into account by anyone, critic or fan, who wants to tell a performer to hang it up.