I have the same concerns. IMO, Biden is a decent person who cares about the country, and he's gotten a lot done. He's running again because he genuinely believes there is more for him to do and because he feels nobody else in the party, including Harris, would have nearly as much success. I don't know what it would take for him to step aside: maybe if he has more falling or other incidents and his physicians advise him strongly to bow out. His wife would likely play a major role in that decision.
If Biden is the nominee, his choice of running mate will assume even greater importance than in 2020. But it would look bad if he dropped Harris, even if he chose another woman, which would be a requirement.
Still, I would vote for Biden over Trump, DeSantis, or any of the other minor players. I don't think an independent could win, and would probably siphon more votes from the Democratic candidate.