I think that a Biden loss this cycle may end up being similar to the Romney loss in 2012. It will be the straw that breaks the camel's back in terms of polarizing the Democratic electorate. "Lost cause Hillary specifically" is an OK excuse, but Hillary is gone, so if the moderate loses again, it will be much harder to spin that as anything other than proof that moderates can't win. I think this worked for Trump, because of what I said above, not just the fact that he was an actual Republican. Trump did stage a takeover of the party, but he did so by staking his territory farthest right and taking moderates for granted rather than the other way around. McCain and Romney's losses proved that tacking moderate was a losing strategy for Republicans. Hillary and Biden's losses might do the same for Democrats.
What scares me about that is moderates are supposed to be the ones winning. When extremists on either end become the only electable options, the stability of the government in question becomes uncertain and political violence becomes more prevalent, as we are seeing right now.