Reading from various angles, I've kinda given up on second-guessing all this. From the last things I've seen, Bernie had the highest support across the party, and maybe in a GE against Trump on a national level as well, but not in the battleground states that are going to determine the actual 2020 election. In the battleground states only, the last polling I saw had Biden up 8 on Trump and Bernie only 3-4, which is damned close to the margin of error. So that seems to be driving the concern.
[I think the national parties' purpose (ideally) -- when the electoral college is still part of our selection of the presidency -- would be just be to ensure that their party has a competitive nominee on the national level rather than having someone selected via state primaries who cannot win a national election -- but between the parties' incompetence and corruption in recent years and then voters losing faith in the party heads, they have been incapable of playing that role.]
Let's face it, you can get as many zillions of Democratic votes as you want in California and it won't change the election outcome. We already found that out. The battlegrounds are going to determine who wins.
That was polling I saw maybe Fri/Sat. So I have no idea what it is now. Things might have already changed. But the chances for a contested nom are way way up.
Whatever is going to be, will be. My concern is more that things are going right where they were in 2016, and the centrists hate the Berners and vice versa, each feeling like each is destroying the country and/or stealing the nomination from the other group. (Honestly, neither of them is my first-choice candidate for the nom, but I know how I'm voting in a GE.) With all the vilification and disgust, regardless of who wins the nom, Trump's going to win the GE if this train wreck keeps up. Good job, team.