It doesnt matter. They arent voting for someone, they are voting against Trump. And even if Biden GETS nominated (I'm not sure he will) and IF he WINS... he will resign within six months for Health Reasons, and the VP will be Pres.
Hell, it wouldnt surprise me if they ran with a VP named "To Be Announced"!
Kind of like, "We have to pass the bill so that you can find out what is in it".
Well... if you want to set up a puppet gov't in America, it makes perfect sense to have a feebleminded (and therefore more easily controlled) man for your presidential candidate. So, for the Democratic agenda, he's actually the perfect candidate.