If Biden were to drop out today, Kamala Harris would be the nominee. Because the most important demographic in the party (ie. AA voters) would be firmly in her camp. Just as it was these same voters (along with those in the Iowa Caucus) who made it so for Obama.
Nobody wants to admit this, but it's true. And, for whatever reason, if he were to drop out today and the party, to the extent that they could even control who ends up the nominee to begin with, were to force her to the side to run Whitmer or others, people either do not understand or are engaging in magical thinking about the **** show that would ensue. To the extent that incumbency advantage exists, that evaporates, and you'd likely ignite a civil war in the party.