Biden was nominated, ran, and was elected president in 2020. He won against Trump, who is not much younger. Since Biden actually has a winning track record (some people think the past is the best prediction of the future we have and Biden won three times, counting the Obama administrations) and since most of the Dems who want to run are ----- unsavory, to put it at the least, all unpopular radical progressives ----- Biden might be the least dirty shirt on the floor of the bedroom.
Biden's successful track record is against Trump. If Trump does NOT run, that throws open the situation, I would think. If another Republican runs, such as DeSantis, the Dems would want to run a new guy, younger, surely? Assuming they can talk Biden out of the White House. He does have dibs on it at the moment.
I would think if Biden does run, they need to switch out the failed VP. Unlike most of you, I don't think there's anything particularly wrong with Biden's mind: he's holding up very well for his age, IMO. But Harris, I really think there is something wrong with her. She at least isn't much of a politician, and that's saying the best I can. Brain damage with perseverance of one word she can't get off is the worst I can say. But there are a lot of other things wrong with her. Suppose Trump declares, as he obviously wants to. Then they (somehow) switch out Harris for some powerhouse experienced pol --- Newsom wants the job, I hate him, too, but he is a pretty --- in that case Biden might well win over Trump, I think. The people want someone with experience and political ability, and they might vote for that ticket.