That’s not showing up in the early polling. The only candidate that beats Trump is the decidedly not-progressive Joe Biden (who carries his own baggage, and is likely unelectable in a general election).
2020 could end easily end up looking like 2004, when the Dems put up an extremely weak John Kerry against a very damaged and beatable George Bush. In that election, Kerry’s main value proposition was “I’m not George Bush”. That’s a hill politicians die on.
There is really no reason to think that a far left candidate can win in 2020. The proposals they are running on poll well when taken in a vacuum (everyone loves a free lunch, after all), but the moment the realities which go along with those proposals (ending private insurance, tax increases, etc) are brought into discourse, their popularity flips.
My read is that the only candidate that could beat Trump *decisively* is one that the ex-Democrat part of his base would flip away from him for. A Joe Manchin would probably win 40 states, but do so with a coalition of traditional Democrats, Trump Democrats, and GOPers who are sick of Trump. That number could get a candidate to 60% of the popular vote.
Trust me, a LOAD of us would like to vote for somebody other than Trump. But if his opponent is talking about Medicare for All (and killing private insurance), or massive programs to address climate change (which we all know would kick up taxes substantially), that’s a bridge too far.