Biden's already been losing the base. I think just changing away from his does wonders for base motivation. No messy questions around Gaza, no age hanging over us, a new voice able to say "we'll do something different" for things that aren't going well.

Plus then we can hammer Trump for being old and incoherent. He wasn't good at the debate (or all his nonsense stream of consciousness speeches), but Biden's jumbled speeches (before this) didn't give us a good footing to be much harsher toward it. "Democrats have chosen a new nominee who will actually be alive in 10 years, Republicans are stuck with an elderly man who slurs his words, forgets things, and talks nonsense about getting eaten by sharks".