I have concerns about Sanders' health, and have reservations about some of his past decisions, but I'd vote for him in a heartbeat before consigning the country to another 4 years of Trump. I think any progressive, or even moderately liberal person, would agree. He will have a VP who is certainly going to be a much better option than Trump or Pence, no?
I can only hope that the most ardent Sanders supporters will do the same if their guy doesn't get the nomination. There is an infinitely long list of prospects that are better than what's in the White House now. Heck, my dog is a better prospect.
Much is made of the "Bernie Bros" who didn't vote for Clinton in 2016 because they preferred to see things burn.
Some existed, but
this was a thing with some Clinton supporters who didn't vote for Obama back in 2008. I can't find data on in-party voter defection to the other party by voters whose top primary choice didn't get the nomination, but I am guessing there is always a certain percentage of "sour grapes" voters who go for the opposite party candidate out of pique over the primary results (or simply because the primary process is so divisive in of itself they believe the rhetoric that the other candidate from their party might as well be from the other).
I don't know what percentage of Progressive voters might be that stupid this time around, but I am guessing it might be smaller, given how truly toxic Trump's presidency has been so far from the perspective of anyone who isn't firmly on board with the entire conservative agenda (seriously, even conservatives should see how toxic it is, but that's another issue).
In 2016,iIt was relatively easy for some to rationalize that the campaign "promises" he made were all bluster and bravado the first time around, and he would settle in and pick a reasonable cabinet and listen to them--after all, he'd been a Democrat once, right? Anyway, it was pretty clear his personal life wasn't aligned with the values of the far right, and he promised trade reforms to "help the working class." I think it was a naive thing to think, but it would be beyond naive to believe it again.
The question is, will spurned primary voters show up to vote en masse for whoever wins the primary, or will they stay home and sulk (or cast ballots with none of the above or hopeless write ins)?
Another question is how many moderates who voted for Trump last time might stay home if they can't bring themselves to vote for whichever Democrat is chosen?
Elections are won and lost based on which group of voters feels that fire in their bellies that makes them brave foul weather and long lines (in states that still make it hard for people who work for a living to vote) to cast their ballots--especially in swing states.