As long as Biden is putting together mostly-complete sentences in 2024, I don't see him walking away from a second term.
Biden has shown that he is still there mentally, nevermind the projection from right-wing authoritarians. And even if Biden has old-age brain because he's fucking old, the fact that he is appointing competent people for positions is enough to show that he and his Chief-of-Staff are up to the job.
If I got to pick someone else to be the 2024 nominee, it'd have to be someone from outside politics who would have a shot in hell of grabbing younger voters who might vote for Trump, DeSantis, Cotton, Hawley, etc...like Jon Stewart, although he has mentioned that he isn't interested.