 Really, the only even semi-plausible answer they've got right now is Kamala Harris. The reason being, if Biden says he won't seek re-election because he's demented, he has to resign the presidency. Doesn't he? If he's saying, I'm not going to run again, not because, like LBJ, I don't want to deal with the politics of it, or I don't think I would win re-election, or I'm just sick of the job.