He is either sick or he is not.
If he is sick and dying—he needs to resign.
If he is not sick, he has been caught in sedition against a President-elect and then, a sitting President, so he needs to resign.
I don’t think McCain has been making his own decisions for a while now. I’m not making excuses for him, but it just seems like handlers and insiders (who lean farther left than he does) took over. These people do not want to give up their power. There will be no resignation. They will use his seat until he assumes room temperature.
Maybe he gets a better funeral or better death benefits if he’s still in the Senate when he passes on? There has to be a reason other than wanting to do in Trump a few more times before then. Well, maybe not.