If the public obviously hates and deplores a particular candidate the parties will presumably pick another one. It's not compulsory but at least it's some degree of considering public input. Which is preferable to the parties simply announcing their single picks and going straight to the election.
It's a game of the numbers.
The Parties aren't stupid. They know full well that 99% of their base electoral is going to vote for whoever the eventual nominee for their party is, regardless of grumbling.
Let's face facts here. We could hold the 2020 election right now with the only two candidates being Donald Trump and "Democratic Nominee to Be Named at a Later Date" and I'd wager the results would be no more then 5% off of whatever the actual final vote is gonna be.
Outside of the extreme margins everybody already knows if they are voting Democratic Candidate or Donald Trump.
So we have two extreme margins. The members of the parties that will either stay home or vote for the other side if their preferred candidate doesn't get the nod and people who are still politically undecided between voting for Donald Trump and the Democratic Nominee, whomever they wind up being.
I don't think either of those numbers are all that huge. But my gut is telling me that second number is just a sliver bigger then the first one and is a better demographic to pander to.
I think the Dems will trade a few "My dream candidate didn't get the nod, screw you I'm going home" votes for what they can expected to gain from undecided voters.
My point, rambling to it that it is, is that "We're telling you who the candidate is going to be, not asking" is what they do now, just with more pomp and circumstance.
The candidate is still to be decided, but that decision is being made by the ~200 members of the DNC, not the ~12 million registered Democrats in the US.
