When Bernie was projected to crush Donald in a general election and Hillary was a coin flip that tells you a lot about their choice of candidate in this race.
As for the primary itself you have a system where the Dems choose who vote and in some states you have people who couldn't end vote for Bernie because of the rules in place about registration (specifically NY). It didn't help that he came in super late and so I understand that he is more to blame than the party itself for his primary loss. That doesn't mean he couldn't have won a general election either. The idea that him losing the primary is some sort of magic bullet argument to his electability in a general is falacious due to the obvious differences between how the two elections work. One system favors the establishment dems and the other doesn't. One probably overstates the impact of black voters compared to a general election etc. Bernie won more open primaries if I'm not mistaken.
As for super delegates the only reason they exist is to pick the best candidate. They did a great job of that :eyeroll:. But in reality voters seeing on every major news outlet huge super delegate lead before a vote has been cast probably has some effect on Bernie voters not thinking he has a chance and there's a momentum effect.
https://en.wikipedia.org/wiki/Bandwagon_effect
As for DWS I don't think she's as much to blame for he primary results as even Bernie himself. But there's a reason she was forced to resign as well.
And for your GOP comparison the problem you have is that all the morons who still vote R in 2017 don't hate Trump. They make false equivalencies or excuses for literally everything he does like Sicatoka. They're the fascist party that will lower your (but mostly rich peoples') taxes and their voters are too stupid to think fascism can happen in the US. The Dems don't have an identity other than lesser of two evils and when the cards are stacked against you in terms of voter suppression and what not, you're gonna need to do better than that.