Bernie has similar unfavorability as Clinton in 2016 but I'm suppose to believe he can beat Trump?
Honestly, I'm tempted to say even rerunning Clinton could beat Trump. Part of the reason she lost was because 1) no one took the prospect of Trump winning seriously and thus certain leftists/independents felt like they didn't
need to vote for Clinton to keep Trump out of office, and 2) people grossly underestimated how bad Trump would be as president, there was plenty of "oh he'll take it seriously once he's in office," "he's just putting on a show for his base, he's really a centrist" etc. that I imagine even drove some of those voters to him. Neither of these factors exist with Trump as the incumbent, he's clearly demonstrated that he can win and how awful he is.
That being said, I obviously wouldn't want to chance it. I still respect Hillary but she's done on the national stage.
The whole thing with Bernie's approval ratings being terrible now speaks to something I theorized in 2016, that Bernie's better polling numbers against Trump were more a factor of him getting to be a hypothetical option for people rather than them confronting the idea of actually electing him. Bernie was looking like the frontrunner for a hot second there, and it wouldn't surprise me if that's contributing to a dent in his popularity. As much as I support his agenda, I'd be lying if I said there wasn't a kneejerk reaction among the population to the kinds of tax increases he'd need to fund his programs, whether you want to blame that on the media, Republicans, "establishment Democrats," whatever (I think people are just kind of short-sighted in general? There are definitely bad faith actors who exploit that), and that was suddenly looking a lot more real to people.
Bernie would ironically be running as a lesser of two evils candidate against Trump, I just think regardless of who that is, it'd be more effective this time around than in 2016.