I never had any opinion about whether Clinton or Sanders would be a better general election candidate. I think people are entitled to have opinions about that, of course, but a big warning bell for me was always people who had *strong* opinions about that. It's unknowable and there are arguments on both sides. Sanders lost. Some of his supporters were and are convinced that he would have won. I think that's silly. Maybe he would have! Who knows?
But Sanders lost the primary. Clinton won. Many of her supporters - and the people working for her - were strongly convinced of the same about their own candidate. That belief - right or wrong - had relevance during the primary. It really had no relevance for the general election. That battle was won, there was no need to keep fighting it. It did not matter whether Clinton was the better candidate, what mattered was making her win. She was the candidate. Continuing to be invested in the idea that she was the best candidate blinded people to ways in which she wasn't a perfect candidate (even if you believed that overall she was the best one). Maybe it was a hangover from 2008. They couldn't stop fighting the primary.
Some of the reasons she might not have been were unfair. Like, you know, sexism. Lots of Clinton supporters in 2008 told me privately (and some people here expressed it in the comments if I remember correctly) Obama couldn't win the general election because he was black. If true, also unfair! It could have been true, though it turns out it wasn't. Racism and sexism are horrible. We live in a horribly unjust and unfair society. Other largely unfair reasons Clinton had problems as a candidate included 25 years of "scandals" that mostly weren't and media treatment of the Clintons generally. Unfair!
But you can't wish unfair away, and you can't make it go away by screaming "unfair!" at it. If Sanders had been the candidate there would have been a lot of unfair things he would have had to deal with, as Obama, Kerry, and Gore did (that racism and misogyny are things which have to be dealt with makes Obama's and Clinton's situations more obscene and depressing, and, yes, more unfair, but no less real). Hell I concede even Mittens dealt with some unfair things. Yes, yes, they would have tried to make Sanders seem like commie-loving Stalinist. Unfair! The man just likes Scandinavia. Still he would have had to deal with that and everybody knew it.
A lot of us thought she would win. Including me! If you think the polls are bullshit you'd better have a good reason to think so, not just a gut feeling. It turns out the polls were bullshit in this election in a way they haven't been before. I'm not sure anyone will ever really know why, or if polls will ever be anything other than bullshit again.
Anyway, you go to an election with the candidate and electorate and the horrible unfairness of everything that you have. The key is to have a strategy for dealing with it. You get no extra points to compensate for the unfairness.
So many of the arguments for why voting patterns in the primary demonstrated that Clinton was the better candidate were transparent bullshit. They were transparent bullshit in part because the exact opposite arguments were made in 2008. I don't fault them for making those arguments to win the primary, I fault (some of them) for apparently actually believing those arguments. Performance in a partisan primary says almost nothing about how you will do in a general election. Bernie Sanders, Susan Sarandon, and the Berniebro army didn't lose the election, any more than voters who stayed home or Latinx voters who voted for Trump did. Clinton, the Clinton campaign, and the broader professional Clintonworld did. It was their job to win. They took on that responsibility. That might be horribly unfair, but that's the way it is.