Biden retires. Harris loses to Cotton or DeSantis.
Cotton is inelectible except in a very right-wing state. DeSantis? He is too new to have established a reputation, so he is hard to pick.
I expect Biden to do what Reagan did that Trump has not done: surrounding himself with good people capable of saying no to him. If he should go into decline, then he will take a ceremonial role whole trustworthy people make policy. Trump surrounds himself with yes-men, and when he goes bad he goes into bigger and deeper trouble rather than backing off.
I am fully aware that people can stay clear-headed into their eighties, but we all know the actuarial issues of aging.