I truly fear Trump has 2020 sewn-up
The Democratic candidate roster so far is very large, but also very uninspiring. None of these candidates seem that they have the capacity to unify and draw in the types of voters who left the Democratic Party in droves in 2016.
As much as I'd truly like to see a female candidate, particularly one of colour, like Kamala Harris, get the nomination, I also suspect that contrary to all the Millennial bleating, only a Baby Boomer, like Joe Biden, can unify an increasingly aging country across the generational lines. If the DNC skews young, it will risk alienating all the older voters who are guaranteed to go out and vote in a way one can't say with respect to the under 40s. Even Bernie, despite being to the left of the Democratic Party, might be a better bet than some of the other candidates thanks to his age and thus his comparative capacity to relate to older generations.
So, please can you bushy-tailed and bright-eyes Millennials get your heads out of your asses, and start thinking about the smart way to beat Trump in 2020: that means supporting a candidate who can engage older voters, as well as the young (who, when they do bother to get off their asses to vote, already tend to skew Democrat anyway).
Bear in mind that whoever the Democratic candidate was in 2016, they were almost due to lose anyway (the last time the incumbent party won a third term in the White House was way back in 1988!) Despite everything being stacked against her, Hillary still managed to get three million more votes than Trump, and had she instead waited until 2020, she would surely have won the EC (in hindsight it's a shame she didn't wait until 2020, and it's an even bigger shame that she's ruled herself out of the running this time). That is why the DNC needs another candidate like Hillary, preferably a woman, better yet a woman of colour, who can win those older voters.