Hillary lost at least in part because of disaffected Bernie supporters. In swing states. Democrats will get more of that. Biden was a fluke because Trump was loathed enough to motivate the progressives that can't stand Biden but to a lesser degree.
Hillary lost because she had 30 years of Clinton baggage and following up a two term president with another president of the same party is really hard. She underperformed in a few key states that Trump overperformed in, largely because of his populist rhetoric that appealed to some traditional democratic voters.
I don't know why it is hard for you to admit that lack of support from the Progressives was a factor.
Fully 12 percent of people who voted for Sen. Bernie Sanders, I-Vt., in the 2016 Democratic presidential primaries voted for President Trump in the general election.
Mark my words, you'd see the same thing with a Manchin-like candidate. Especially if it was a less buffoonish Republican. You think the progressive voters are going to forget that he destroyed voting rights protections, social safety nets, and other issues dear to their hearts?
I donated to Sanders, not to Hillary and not to Biden. I donated to Gary Johnson. Fundraising takes a toll that you're not going to make up with centrists in my opinion. Possibly you might get a mass influx from corporate donors who want no part of progressive candidates.