Corbyn is unpopular for many, many things not just the antisemitism. Except being anti-imperialist is not why people in Labor didn't like him, it;'s how he does that that alienates people. You're either on Corbyn's team or you're gone. There is no compromise with Corbyn. Nobody had to do anything to make Corbyn look like an anti-semite, he did that to himself. It's one thing for the right to do this, it's quite another when he fails to address concerns within Labor itself. It's true Bernie's more liked than Corbyn, but that's because he's never been in a position like the presidential race - the risk he was compared to Corbyn is that he might have got a loss as badly as Corbyn's, and Trump didn't need to have that to win just demotivating enough voters would do that. The right would have got another angle on Bernie, it's not like he's bulletproof. All they need is the Communist canard, and that would have sway with voters in both parties. Not because he's a Communist but because he's given ammo to his opponents for years that they can mine ad he continues to do this even during elections. It's why they wanted him over Hillary.
Primaries are practice for the presidential race, and he has trouble making the same mistakes when he's running for election.Anyway, the primary performance is irrelevant. Had Sanders or Warren made it to the general election, I don't think you can rule them out from winning.
Self reflection and analysis is crucial for politicians, it also shows that they're willing to be humble and prioritise strengthening their weaknesses. A challenger who fails to do this sends a bad message about how they govern if they won because if they're not going to do that running against Democrats how are they going to fare with real power with millions of lives at stake? You know what politicians need to win elections? Not do everything possible to alienate the voting blocs they're trying to get the votes from. Bernie is really bad at doing this. Bloomberg was more in tune with black voters than he was! Imagine if he said that Castro comment while campaigning against Trump, been who he is that was a risk to Democrats we didn't have with Biden. I agree about Warren.Remember per Allan Lichtman's Keys to the White House, an election is a referendum on the Incumbent and the Incumbent's party, so the challenger's campaign matters little except for "Challenger charisma" and Bernie is more charismatic than Biden, so he'd get one key more than Biden in that slot. Warren probably not. Since she's too much of a wonk to work on the campaign and while she does great in showpiece debates (like destroying Bloomberg's political future), she wasn't able to articulate that to her base.