Plan B for many of them is a brokered convention. I’ll have to do some reading about the new convention/delegate distribution rules. I think they have been changed since 2016 though I would guess not by much. The DNC rules are designed to give the appearance of democratic principles but essentially give all the power to decide the nomination in the backroom with the “super delegate” vote.
The GOP is a little different, it has super delegates too but not nearly as many, and the GOP primaries are winner take all for the states. In the DNC, they allocate popular vote proportionally by state, so it is possible that someone who comes in 2nd or 3rd or even 4th in every state - who never got a majority in any state - could become the nominee if they swing the super delegate votes. Harder to do that in the GOP.