I suspect that this has already passed. With Jobs having announced his leave of absence, indefinitely at this point, I think the process for him to step down and accept a lesser role has already started.
I think you are right, whether SJ is truly too ill to continue, or that he is OK but genuinely wants to retire as CEO. Personally I hope it's the latter and maybe his survival of cancer was a wake up call and that, while he's in remission, it makes sense for him to retire and enjoy what life he has left.
Whether his death is imminent in the next year or two, or if he has decades left, he's not going to live forever. Unless you are in your 40s, 50s, or older, it's pretty hard to convince most younger people of just how time appears to go by faster every year and how you enjoy your life becomes so much more important.
If I was in my mid-50s, worth a few billion and well regarded for a turnaround for what was once a dying company, I would be satisfied with both my achievements and my bank account and retire. I would, of course, find ways to help others with my money, ala Bill Gates, and even have a foundation, but I would seriously cut back on work that took time away from family and friends.
The other thing I think is amazing about SJ is just how terrible he was as a businessman in his first tenure, and how most regarded him as creative, artistic, but never a businessperson like Bill Gates or Michael Dell. In his second run at Apple, he kept his creativity but acted in a businesslike way which rivaled anybody in high tech. Suddenly, he was among the best chest players in the game.