I'll take your word about Biden's decline, but in 2020 he provided a kindly/sensible foil to Trump's mouth-foaming in the debates. Short of Disney creating an animatronic Biden, we might need to:

1) Have Biden run just close enough to the center to not temp the lefties to run their own candidate.

2) Switch out Harris with Michelle Obama or Condi Rice, if the latter Biden may need to run more to the left.

3) Give Biden an earpiece, cue cards, and a shot of sildenafil prior to the debates. Besides his own simple talking points, he just needs to "Aw c'mon man!" after all of Trump's rants.

4) Once he's elected, it doesn't matter (see Reagan's 2nd term)