As a caveat to what I said, there's also the possibility that the Democrats would do better moving away from the 'checking minority boxes' approach that a lot of people, at least online, seem to be advocating. This isn't to say they should nominate someone like Cooper or Hickenlooper or Murphy because he's a white man, far from it, but that it might be behovely to let such a person through the primaries if it looks in three years like he'll be the best messenger to take the party back to power.
But most primary voters don't think strategically like that. "Checking minority boxes" may well be closer to what Dem. primary voters are thinking, and thus it may determine the nomination, even if it's not a smart way to go for finding the strongest GE candidate.
For example, let's say Cory Booker runs in 2020. I don't know if the fact that he's black is going to mean that he'll get the overwhelming majority of the black vote in the primaries, a la Obama 2008. But it's certainly a possibility. And given the way the '08 and '16 primaries went, it seems like dominating the black vote gets you a long way towards winning the nomination. It might not be how voters should think if they were to act strategically, but I don't see a way to stop them from doing so.
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