I agree polls can't be taken seriously at this point. Let me amend to: If Sanders wins the nomination, I would need to see some pretty robust polling at that point in the process to be convinced.
I hope you are correct that Bernie can win over swing state voters, if he wins the nom. I just think the socialist label is going to scare them, along with the Dems' generally more pro-immigrant platform, and the open threat to take away the health insurance that people already have and give 'free' things to people (it doesn't matter that those policy promises will not happen; it will still scare them). I'm not at all sure that Sander's less internationalist, more protectionist economic messaging will overcome those things. I keep thinking of my father, who is a conservative leaning independent in one of the states that crucially swung for Trump. He voted for Trump (b/c for random reasons that he couldn't articulate he just hated Hillary, despite voting for her husband and also for Gore back in the day). He's regretted his vote for Trump since about a month after the inauguration and bitches to me CONSTANTLY about how much he wants Trump removed from office and what a threat he is.
BUT I think it very unlikely he will vote for any of the further left candidates the Dems are floating. And he absolutely won't vote third party b/c he views it as a wasted vote. I suspect he'd vote for Klobucharr or Gabbard or any boring Dem centrist (though probably not Buttigieg b/c he's old and kind of homophobic). I'm pretty sure he'd vote for Bloomberg in a hot second. I am very skeptical that he'd vote for Bernie or Warren. In fact, I suspect he might vote for Trump again (miserably) rather than vote for either of them.
But who knows, it's one data point. I keep getting visions of a Goldwater style blowout if Sanders were to go up against Trump, but the vast majority of the country's votes are likely locked in anyway already, so that's probably not likely.
Problem is, I'm a cynical bitch who expects nothing good from humanity, so it colors all my expectations. Y'all keep on believing, and I'll vote for whomever the Dems put up, even though I'm totally un-enthused about any of them.