IMO, no that is not the best move. There is no one prepared. It's too late. You are saying that without saying who it would be. The incumbent is still our best chance. Warren, Buttigieg, Harris, or Whitmer wouldn't win. And not having any time to vet someone could go sideways fast (thinking of Edwards)--not saying this would happen with any of the people I listed but I'd rather go with who I know has already beaten Trump. Also, figure out who of that list would actually WANT to step in. Why give your one shot of being the candidate now? Under these circumstances? If you have presidential ambitions, that's a gamble.
Remember, Harris had to drop out of the Iowa caucus because her polling was so bad, she didn't stick around at all. She had a 3% polling for supporters, tied with Andrew Yang, Tom Steyer, and Tulsi Gabbard.
My opinion, here is what should happen
1. Biden continues to make his speeches, which he has already started to do so, that showcases he is together and can even be a little fiery. He doesn't have that "cold" ever again. His staff has a clear directive on what needs to be showcased and don't "overprepare" him, but rather laser focus on attacking Trump and touting what he has accomplished.
2. Trump's felony sentencing happens. Attention is drawn to that side of it again
3. Trump finally picks his veep. I've seen that shortlist. Nothing super surprising but at least his other veep was in the traditional Repub fold. Could be another avenue of attack
4. The Dems send out any all-stars that can help stump for Biden. It has to be very careful. Figure out who would be best, where, and when. Whether that is our past nominees, Obama, maybe even Bill etc? (not sure, don't ask me, I don't know, but hopefully they know) or more local people like Abrams, governors, senators, etc. Hopefully people who are charismatic who can explain his accomplishments and the danger of Trump.
5. The September debate happens. Obviously everything here needs to happen better for Biden. Perhaps also the format or moderation will be better suited for him too. This is also assuming the debate happens
6. The veep debate happens. Harris is not loved but at least here she can tout that she is competent and not a danger to democracy.
7. Likely some kind of October surprise. Repubs might do something around Biden's son again. Maybe something in court on Trump's side. Etc etc
8. Likely what will happen is a lot of people won't vote, they'll stay at home, frustrated by the options. More than usual. But maybe that will hurt Trump more than Biden. Trump pissed off a LOT of people, and that can be a stronger feeling than reservations about a guy's age, especially when he has a veep who can step in if need be.
Is this perfect? No. But I think this is our best chance.