The assumption here seems to be that Bernie voters will be so ticked off if he doesn't get the nomination that they'll defect in huge numbers voting third party, voting for Trump or just not voting at all. Result: Democratic defeat. It's kind of like that old National Lampoon cover: "Buy this magazine or we'll shoot this dog".
I think people can calm down about this one. If Democratic voters refuse to "buy" Bernie, I think it's quite unlikely there will be mass defections. After all, in 2016, the defection rate of Bernie supporters--who you may remember will apoplectic by the end of the process--was actually relatively low: according to CCES data, 17 percent of Bernie supporters either voting third party, voted for Trump or didn't vote. That compares to Clinton supporters in 2008, 30 percent of whom wound up voting for McCain or not voting.
So, defections by supporters of a losing candidate in a contested primary do happen. But there's no reason to panic about how deep the defections will be if Bernie doesn't get the nomination this year. They're not really gonna shoot the dog.