If Team Biden can't allay the fears he is too old, voters will be implicitly pricing in Harris taking his office during the next term anyway. So you either comprehensively demonstrate the "Biden old" stuff is bogus, or if you can't (because true) or can't (because incompetent), get out of the way.
So we move to Harris now and fight this issue right up front. Bonus: Dems get to spend 4 months campaigning on how old, infirm, and addled TRUMP is. If elected, at half-term he’ll be the same age Biden is today. “We fixed our old-man problem — but the Republicans have not.”
