I don't think the left is going with the Booker/Harris coalition in 2020. The Bernie/Warren voters are younger and I could see a hard swing back leftwards in the Midwest with Warren or Brown as nominee. The elitist wing of the Democratic Party isn't terribly large. If you take the Bernie voters from 2016 and add more minorities, it's a clear win for the left.
The idea that there is this huge base of far left people in the rural Midwest who didn't vote is a fallacy. Unless Trump is very unpopular (which is very possible of course) I think it's more likely that a far left candidate just repels the gains Hillary made in places like the Milwaukee suburbs.
