Does he lose his healthcare insurance coverage if he resigns from the Senate? I wonder if that is a factor for him.
Interesting question. They used to get their insurance through FEHB, where all civilian feds get theirs, but that apparently changed after ACA was passed, and the senate and congress had to start buying on a special exchange. I think they still pay the same 70%gov/30% employee portion, but I don't know how the rest of it works.
Though I'm sure McCain would qualify for TCC (fed version of COBRA), which would give him 18 more months of insurance that he pays himself after he separates, and it sounds like that would be long enough, given his prognosis.