Lets say we arrive at 6 June 2020....the last primary of the season...with five weeks to go to the convention (13 July), and it's now become obvious that Joe Biden has a minimum of 2000 delegates, which logically would crown him as the primary candidate for the party. But let's add this scenario....that Joe is mentally not alert enough for the job.
Could the party enact some exception and void the first round? In theory, they could make up such a rule....if this was the business of day one of the convention. So this would open up the entire convention to a new candidate out of thin air? Yes.
I'll admit.....it would be a crazy idea, and would require a majority of the delegates to pass such a 'deal'.
Destruction of the whole process for 2024? That's really the hurtful part of this idea, why bother running if such a rule could be invented out of thin air.
So this is where someone like Bloomberg or Hillary Clinton would appear? In theory, yes.
The problem here is that appearance that Joe Biden makes.....is a fifty-fifty shot at no dementia issues. I would make a humble guess that one-third of Democratic voters are dismayed by the Biden-problems and can't really form anything positive about the election.