If Biden resigns (soon!) and Harris then is our nominee, nearly everyone who would have voted for Biden will still vote for Harris. So will millions of under-40s who are unenthused about someone their grandparents’ age who is obviously declining physically even if he shows good strength and energy some of the time. The polls have been showing this weakness for months and it is real. Younger people energized by a Harris candidacy will easily outnumber MAGAs who would have been non-voters until Harris comes along.
We get a generational change and advantage when the R’s are trapped, stuck with a much-too-old monster who they can’t come together to replace. No matter what dirt the R’s throw at Harris, it won’t win votes for Trump and it probably gets Harris some sympathy votes in addition to the generational energy thing.