So far, it has looked like Bernie doesn't "need" the youth to win only when there's a super divided field. We'll find out after today, though! If Bernie runs up huge margins today then he'll put the "electability" conversation on ice (which would be nice).
The only way to prove you can build a large coalition that will deliver votes is to build that coalition and deliver those votes.
As for the suburbs - there are a variety of data points to consult. Biden polls best with the older, "moderate" voters that populate the districts that flipped in 2018. Here in Virginia, for instance, (a) Biden is polling the strongest, (b) the districts that flipped in 2018 all did so with "moderate" candidates that appealed to embarrassed Republicans, (c) those new politicians, in turn, support Biden because their constituents are milquetoast suburban white people who work in the gov or military and are scared of ""'socialism""" or whatever.
Here are some examples of what I mean. A quote from the second link:
"That concern about Sanders in a historically moderate state was reflected in the poll, with 67 percent of respondents saying they are worried that the eventual party nominee might be "too liberal."
Like, I see more Bloomberg signs around than anything else, which should tell you something.
I was just adding to the discussion, apologies if I derailed anything.