There’s a kind of a chicken-egg argument here. You would probably say that the small minority of “Bernie-or-bust” people who either stayed home or voted for Stein did so because Clinton was too moderate/uninspiring. That may be part of it. But I would argue that a lot of those people probably read too many nonsensical lies on forums like this one that essentially said “Sanders is the only true progressive and everyone else is a centrist phony.” That’s why continuing to perpetuate this absurd myth of Sanders as our one true savior is so dangerous.
The idea that voting might not even be worth the effort in an election between a moderate progressive and an ultranationalist is a horrifying delusion we need to be pushing back against with all our might. Elections have real consequences for real people. Choosing not to vote because your personal hero isn’t on the ballot is a privilege many people don’t have. The most vulnerable people in society tend to be the ones who get that.
I don’t dispute anything you’re saying about barriers to voting. Obviously, no Democratic candidate is going to flip a deep red state, so that seems like a moot point. The question is who is more likely to carry the swing states. Where is the evidence that Bernie alone can do that? Biden is polling slightly better in most of those states right now, has consistently been the most popular among African Americans, and has the most cross-over appeal. I don’t think Bernie winning in those states is impossible (maybe he could bring back some of the delusional Jill Stein voters), but where is the evidence that he has a significantly better chance than Biden? Are 20-something democratic socialists that big of a voting bloc in the midwest?