what happens if warren drops?
well, what happens in strictly literal, blunt terms is that the key swing demographic that won them the 2018 midterms ends up disenfranchised. they're not going to like any of these options, and they may end up staying home or voting republican.
i can't imagine many of these people voting for joe biden, who they're mostly likely to interpret as a dunce. and, they're afraid of sanders...partly from the propaganda, but nonetheless...
warren was the weakest candidate in the field, but she's also proven the most stubborn, and the system is likely going to reward her bad behaviour.
but, at this point, does it help sanders or biden if she drops? it probably helps trump. i know i just flipflopped, but the options just changed. and, you want her on the ballot to give these swing demographics somebody they can vote for so they can be dragged along, even if she doesn't have an actual chance.
if she drops early, and the voting pool ends up restricted to leftist revolutionaries in the sanders camp and low information voters in the biden camp, if educated voters don't have anybody to support and tune out, it's going to undo the results of the 2018 election.