Your theory that party elites were mainly concerned that Bernie would lose to Trump is belied by the fact that they were clearly willing to make a move that would almost certainly hand Trump a win if it meant stopping Bernie from getting the nomination.
Read the article that's linked in the text. The New York Times interviewed 93 superdelegates. The vast majority were willing to nominate some random Democrat if Bernie came in with a strong plurality but not enough to win in the first round. Some names they floated included Michelle Obama and Nancy Pelosi, who is consistently one of the least popular politicians in the nation.
Bernie Sanders is consistently one of the most, if not the most, popular politicians in the country. You're making a lot of assumptions about political behavior that are unsupported by the facts. The vast majority of voters tend to vote consistently for one party or the other. Swing voters constitute about 10 percent of the electorate.
The assumption is that these folks are politically moderate when in fact they are politically weird. They're unpredictable. Generally speaking they're disengaged and uniformed. They often vote based purely on personality and other things that have little to do with politics. And if they are going to vote for policies, then they'll favor those that speak to immediate material interests, i.e. jobs and healthcare.
A study on Obama-Trump swing voters found that three-fourths of them were dissatisfied with Obamacare, so they'd be susceptible to a pitch for single payer, which was the centerpiece of Bernie's campaign (for reference, see his Fox News Town Hall, where he was able to make the case convincingly for single payer and won the support from most of the audience).
Lastly, you can win elections by trying to guess what weirdoes like Ken Bone want or you can expand the electorate by mobilizing more groups that traditionally don't vote proportionate to their demographics (the poor, youth, non-white people). You do that with grassroots organizing (that's literally how Georgia was flipped this year). Bernie had a million volunteers. Biden had a skeleton crew.
The real power in the party, which is to say billionaires like Donald Sussman, didn't want Bernie because he would threaten their material interests. They would be perfectly fine with Trump.
