If Biden drops out of the race, the candidate would be Harris. He'd resign, endorse Harris, and it'd get confirmed at the convention. The only "fresh new, younger face" would be the VP pick.
I'm not saying that's a problem, but the idea that that an open floor battle at the convention would be a good idea is nuts, and just discarding the sitting VP like that would probably shatter the democratic coalition.
Assuming Harris wins, the first midterm federal election is usually ugly for the president's party, so it'd be a risk. Especially coming off of this election where dems will have to be extremely lucky just to hold onto the majority (even with the vp tiebreaker).