The dude's in his 60's. I'd love to believe he'll go on for another 20 years, but it's just a fact that the body takes longer to recover from illness the older you get, doesn't always recover fully anyway, and he got taken down hard. Also, he put Yes on hold so he could go do his solo tour and mini-tour-thing with School of Rock, meanwhile the rest of Yes was ready to tour again and hopefully even work on new material. Then he got sick. He now claims he's still too sick to tour, but not too sick to do more of his solo shows, because those aren't as stressful. The others got tired of waiting for him, found another singer, and went on tour anyway.
All of this has caused possibly irreparable damage to relations within the band. The others consider Yes a band, with each member having a say in the decisions made. Anderson clearly sees himself as the leader; in his mind, Yes is his band, and he requested that the guys not tour without him. He said it wouldn't be fair to the fans who would be expecting Yes, when it wouldn't really be Yes because he wasn't there. Then he went and did some more solo shows.
It's been years now. Their last album was eight years ago. If Jon truly wants to get well, well enough to tour again with Yes, he'll wait and rest and hopefully recover. I understand that he's a performer and can't resist the call of the footlights, but with his advancing age and failing health, he cannot have it both ways, and honestly he's just being a selfish ass about it. They're touring; he's literally playing with himself.