This is an absolute shitshow for the Dem party. I'm definitely no expert on elections, delegates, etc., but I know it's a lot more complicated than someone simply declaring a brokered (or contested) convention this summer. Pledged delegates "must reflect the sentiments of those who elected them," and the primary ballots are already set in stone for the Super Tuesday primaries and other primaries later in March. Newsom, Whitmer, Kelly, etc. aren't on those ballots now and can't be added later as the deadline for filing has already passed. Also, it's important to note that none of the people whose names are being floated have campaign organizations in place, which is an expensive, labor-intensive and time-consuming process.
Again I'm no expert, but my layman's understanding is that the Democratic Party has two (and only two) options if they're serious about promoting someone other than Biden: 1) Immediately rally around another candidate and quickly mobilize a write-in campaign for the new guy or woman with respect to the upcoming primaries; or 2) since filing deadlines for independent candidates are much less stringent, the party could promote someone who runs as an independent. That would be kind of crazy and unprecedented, but these are crazy times.
Both of these options seem like longshots. By the way, I don't know that they have additional viable options even if Biden were to announce tomorrow that he's changed his mind and is no longer running. As others have noted, millions would perceive Harris as the heir apparent over the strong objection of millions of other Democrats. You're then looking at a completely fractured party just months before the election.
I'm not sure what they can do other than hope Trump is tried and convicted on the election subversion case prior to November (polls show that a conviction could very well be a game-changer) or that he shits his pants on a stage somewhere. If you're keeping score, the criminal charges tally is Trump 91, Biden 0. Maybe Biden would have fared better if the Special Counsel had recommended criminal charges yesterday instead of saying Biden's memory is bad. Crazy times.