Maybe you answered the question I was about to ask, which was "what has this guy got to gain by selling out at his age?"
“One word: Dementia”
I almost wish it were so, but it is not. Last election I observed Senator McCain closely for about an hour. He was sharp, knew what he was talking about, and at the top of his game.
The only reason that I can come up with is that he is a lifelong progressive.