Who said they're the only ones that matter? Of course they're not. But they are the people the Democrats need to win back in order to beat Trump. These people went Obama both times, they went Bubba Clinton, but didn't go Hillary. That's a problem, one you don't overcome with someone in-line with a Berkeley student in terms of world outlook.
2016 says otherwise. They're why you lost, you've already got the progressives on board. Don't expend effort on courting the Bernie Bros, they're not going to sit home a second time and let Trump get a second outing. They've learned that lesson.
I mean, again, if that were the case Trump wouldn't be in the White House. Millennials are a huge piece of the pie, but they're not a unilaterally dominant factor just yet, not when enough of the rest of the demos aren't on the same page.
Risk it, by all means. You run a Warren, you're getting another 4 years of losing your minds over Trump. But hey, millennial prerogative I guess. Personally I don't even think Biden will run now, at that age who needs the trouble, but aside from Harris there's also not really anyone else in the race who has a chance yet. That could change of course, really early days, but so far it's pretty evident the party still doesn't quite grasp what went down last time around.