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.
TBH if I were convinced that Biden is already too old, that he's as out of it as he appeared at the debate all (or most of) the time, I would not be pricing in Harris. In that scenario Biden is either muddling through with diminished capacity or being "guided" by aides, and would continue as such.
