What I fear is that many Bernie supporters will stay home and pout if he doesn't win the nomination.
I love how people say this and everyone just lets it go but if I suggest that catering to the extreme Left Twitter/Tumblr crowd is going to cause people to stay home all of a sudden I have to empirically prove it and have a 50 page Power Point presentation ready with "how I would fix it."
Unless the rust-belt blue states that lost the Democrats the election in 2016 have turned into beacons of Progressive Extremism and nobody told me I think hand wringing over losing a few Bernie Bros at the expense of actually making gains among moderates is politically suicidal.
The Dems are going to win 2020 by making gains in places they didn't win in 2016, not in making the places they won (i.e. the liberal, progressive urban areas, West Coast, New England, etc) like them more.