McCain and Thad Cochran are drooling doddering dinosaurs who rely 100% on their aides, to find the cafeteria, the restroom, and their desks. As long as they can be coached by McConnell to press the proper buttons in the Senate chamber, they'll be serving as long as they want.
You are 100% correct! McCain was a loose cannon for years, but at this point he is nothing more than a puppet who is used by the establishment for whatever purposes they deem necessary. His only motivation that I can tell that keeps him going on is a desire for attention.