Are purely hypothetical questions allowed?
Mostly no, but also yes. :-)
Purely hypothetical questions are likely unanswerable. For example, this question posits such a far-fetched situation that most answers would be wildly speculative and not terribly useful to others.
On the other hand, some questions are technically hypothetical, but still answerable. To use my own example, this question about doing research without an IRB was not inspired because I had personally transgressed; rather, it was a topic I became curious about after reading some other posts on this site. And I think it was a good question.
So, what should happen with this question?
This question does seem a bit poorly-motivated because the professor is already on the committee and there's nothing to do except wait. And if he does die, the way it will be handled will vary widely from department to department. There's no real decision we can advise on; we would only be trying to predict the future.
On the other hand, consider this slightly different question: "I am forming my thesis committee now. I want to include Dr. Bob, but he is 80+ and in poor health. If I include him and he dies, is this likely to delay my graduation?" This one seems eminently answerable -- it's a real dilemma OP is currently facing, and while the handling will vary from department-to-department, we can still give a general answer.
Given this, I personally would leave the question open. While the current formulation is not so good, it is "isomorphic" to a well-formulated question, and I would not want to split hairs. That said, I think voting-to-close is also defensible.