The biggest argument against removing Biden from the ticket against his will is that we went through a bunch of primaries, and he has won enough delegates to be the nominee. For the party that claims to be about protecting democracy, it would be a stretch to tell all those Primary voters “Hey, guess what? Your vote doesn’t really count” so directly.
But, there are contingency plans for a reason. If Biden’s doctors decide he has a sudden case of chronic not-gonna-win-itis that is affecting his health, and he gets a sudden medical diagnosis that forces him from the race (and the Presidency), there are precisely two people who can take over without the Democratic Party totally abandoning the democratic process:
the newly minted 47th President. Kamala Harris. While she did not get any delegates herself, as Biden’s VP she shares the ticket and would take over the Presidency once he backs out.
Dean Phillips , who as far as I know is the only other candidate to get any Primary delegates. No, “uncommitted” doesn’t count.
Nobody else can swoop in and take over, because they have no legitimate relationship to the Biden ticket, or to the primaries.