Imagine you aren't a U2 fan, if one of your friends, the one you can't ever doubt of, tells you he's been dining with Bono because he's paid him for it, what would you think of Bono?
Last year while U2 were rehearsing in Barcelona, this guy went to Mallorca, he got into a famous restaurant and, as nobody was paying him attention, he asked for a guitar and started singing, then he let people took pics with him and someone paid for his lunch without saying he was an impersonator, when I saw the pics some days afterwards it was a hard job to get my friend believe it was not Bono, only after looking at the pictures in Camp Nou she had to admit she had been cheated.
Now, imagine what others may think of this: Bono has such a big ego he gets into a restaurant makes everybody notice his presence imposing himself with a guitar, and he sings terribly without all the technical devices (because Pavel sings terribly).
On the other subject, I believe that if any of the U2 members has or decides to retire, the whole band will retire too, we could expect some little collaborations, especially Edge and Bono doing some public appearances for charitable initiatives and I hope they will continue writing music after they decide not to tour anymore.
Personally, I don't mind waiting for Bono to recover or for any of the members of the band if it is the case, but if I see him replaced by someone else I won't ever buy any of their music or go to a concert again.