Guys like B.B. King and Buddy Guy are really the last men standing of their generation. I've seen plenty of mention of their contemporaries like Freddie King, Albert Collins, Bobby "Blue" Bland and others in this thread. And you know what? They're all gone. The same as Muddy Waters, Howlin' Wolf and T-Bone Walker. And since I saw him play with Hubert Sumlin in 2010, Hubert has passed, too.
B.B. is notable for how he plays. His vibrato, tone and phrasing are all his own. Nobody else sounds like him. And when you hear him you know it's him. Establishing that kind of identity in music is a tremendous feat.
I think it's fair to say that he's past his prime. He would probably say that too. But in the same respect, he's part of a living history of the blues that stretches back to his first record in the 1950's. And when he's gone all we have is the music and the impact he's made on countless musicians.
One thing that's noteworthy about B.B. King and his career? He was one of the first bluesmen who was able to cross over. Because in the 1950's the blues was considered race music, and was only marketed to black audiences. Oddly enough B.B. was able to cross over because of Frank Sinatra! Sinatra recognized just how great B.B. was back in the 60's and got him into white clubs in Las Vegas, and that opened a whole world to his work.
All of that being said, I think B.B. deserves respect for what he's done over the course of his career. And for anyone, I think that knowing when and how to bow out gracefully is important. But when you look around and almost all of your peers are gone, like B.B.'s are? Retiring from touring would probably seem like you're getting ready to call it a life. In that regard I can't blame him for wanting to go on.