The democratic primary is not a mini version of the general election, there’s a different target audience and different factors. The democratic primary also does not favor people who are barely in the party, like Bernie, but rather it favors people who are well known within the party, like Clinton. Acting as though Bernie had an advantage in the primary doesn’t hold up, and acting like you couldn’t win the presidency if you can’t win the primary also does not follow.
For a simplistic example, imagine there are 2 groups of voters. The democratic regulars will vote democrat no matter what, and they tend to favor people well known in the party who are in party leadership. And a group of people who are usually not interested in politics, but they are interested in Bernie because he’s a rare person not beholden to corporate and rich interests above all else in our managed democracy.
Now - you run a primary with those candidates, and the well known democratic candidate wins. However, what happens if other candidate were to become the nominee? All of those people who believe “vote democrat no matter what” are still going to vote for the guy, but you also pick up all those voters who aren’t regular democratic voters but who would vote for him. That number combined is bigger than just the regular democratic voters. In this example, the guy who couldn’t win the democratic primary still does better in the general election.
Now, you can make another case that he alienates moderate voters that blah blah. I’m just saying “he can’t win the primary therefore he couldn’t win the election” does not hold up. Those are two separate contests with different factors.