I truly do not believe that a minority or a woman will win the primary. I think a majority of Democrats are going to get on the "who is electable" bandwagon, and a straight white male will win the nomination. Because its safe.
Nobody in the Democratic party wants to lose the 2020 presidential race. We need someone who will get out the vote. We need someone who will not alienate anyone by being "different". (Or evil, or a sellout, or whatever else they called Hillary). Straight white males do not automatically alienate voters. Anyone else is "risky".
The polar opposite of Trump would be a thoughtful, progressive, educated candidate who can make promises that resonate with the Democratic base. He will probably be charismatic. Yes, he. He will probably be either a senator (probably white) or governor (white). In fact, I am sure that he will be a seasoned politician.
Kamala Harris is risky. It will NOT be her. Cory Booker has some skeletons that people will play up. It will not be him.
I don't think anyone knew that Clinton or Carter would win. But people seem to like governors. There are no black governors. Not even sure if there are Latino governors. Or women governors.
But yes, that's my theory. The opposite of Trump is someone with a brain instead of bucks. But the Democrats will not run any risks in 2020.