How do you, if you believe in Bernie's policies not vote for Biden over Trump...?
Your forgetting their other option: spending their Tuesday doing anything other than voting.
Sorry. I find that attitude of indifference reprehensible, to be quite frank. Feel free to disagree.
I think it is possible to both find an attitude reprehensible at the same time that we acknowledge that it (spending the first Tuesday in November doing anything but voting) is an attitude shared by almost half of registered voters in the USA.
If "I don't care enough to vote" had been on the ballot it would have defeated both Clinton and Trump in a landslide in 2016.
I'm not arguing Sanders is the solution to that, because I don't think he is.
But I don't think it makes sense to disregard the fact that tens of millions of americans clearly cannot be motivated to vote for candidate A simply by reiterating how terrible candidate B is. Regardless of how personally incomprehensible (or offensive) you or I may find that worldview.
Actually, reiterating how terrible candidate B is works really well for the Republicans. That's basically how Trump got elected. Everything Dems/Hillary do is either outright bad or a hoax/plot/conspiracy even if it doesn't sound like it on the surface. And their whole goal is to fleece you/replace you/actually outright kill you via hordes of raping Mexicans.
But I think (at least part of) why this works for Republicans and not Dems is that the Republican party is so predominantly white Christians and older and more male. It's easy to hit on the thing(s) that scare/anger white Christians in simple, easy messaging. Dems are liberal white Christians, conservative Black Christians, every other faith and none, all the other races and education levels and every other "identity" you can think of, including a handful of old white Christian males. It's hard to get such a broad coalition to all be scared/angered by the same simple messaging. The interests are too diverse.
There's a lot of research going on now that negative partisanship is one of the most effective tactics, and that turnout in 2018 was high significantly due to negative partisanship. The fact that it is always higher in the mid-terms by the opposition party is a testament to negative partisanship and 2018 was a banner year for Dems for that. But turnout was also unusually high for Republicans in 2018, perhaps bolstering the argument that they are easier to stoke with negative partisanship than Dems are. Rachel Bitecofer has a lot of fascinating research about this, and Ezra Klein's new book is really informative.
It may not be very flattering to the human species that negative partisanship can be so effective, and it goes against my own ethics to think about using it (instead of winning on the merits) but I'm also tired of Dems bringing knives to a gun fight. If they could leverage it as effectively as Republicans, far be it from me to insist Dems snatch defeat from the jaws of victory.