Of course there isn't. It would be a hot mess of people.
And as you stated is the million dollar issue, which is why 'the experts' were saying Biden had to figure it out last March, to give whoever is to be the one enough time to run a campaign, get known nationally.
The issue with the Dems that makes it harder is it's balancing many different political desires. Say what you will about the Republicans but their, ah hem, less diverse constituency, benefits streamlined political stances.
Well now there's the Maga/Rino break but until then, you didn't have to balance your messaging as much as Dems do.
Take Israel. It's pretty easy to figure out Israel messaging from the pubs. With the Dems you risk pissing off different factions (and make no mistake, there is a larger than normal Arab faction in Michigan that are super pissed at Biden).
Anyway, to find one person who is a decent dem unifier at this late of the hour will be a nightmare and make the party look weak.
Which is why....if Biden is really rapidly deteriorating now that's just the worst case scenario from an electability pov.
For the record, I believed he was a one termer as part of the process. The old man called out of retirement to kick Trump out and reset the table. He was Clint Eastwood in Grand Torino.
If he ends up dying or having his brain brake during this stretch run than, shame on us and everyone around him.
As you said you don't force out an incumbent but, you do maneuver it in a dignified way if there really is something wrong.
We're stupid but I don't think we're THAT stupid.