Also he's gotten too old. He's literally becoming dysfunctional at the role and I am sure knows it.
There's also no particular consequences for the party of him stepping down from leadership at this point -- it's not like he's going to lose a bunch of senior committee assignments for the GOP, for example. It isn't like with, say, Feinstein, where even though everyone thought she needed to go, they also knew that having her leave would be a disaster because the psychopaths across the aisle would refuse to let anyone else take her seats.
I doubt age factored into his decision. He was too old ten years ago. If he had kept control of his party, he'd be clinging to power all the way to the morgue.
The consequence is that he is not a total simp for Trump the way most of the rest of the party is. Aid to Ukraine has been a very large division, to name one example.
He might be less enthusiastic than the others, but any leader who is not clearly and directly rejecting Trump as a treasonous criminal is under his thumb. SNL had the right take.
And by retiring from leadership before doing it, he's now lost his chance to do it. He supported the man until the end of his career and now his career ends. Same as Romney, it's now too late to make amends.
My point isn't that he's a good guy. I'm saying that he's not Tom Cotton, and if you don't think that's a meaningful difference, you don't pay much attention to the Senate.