Democrat primaries pump out moderate Democrats. They generally have since 1984
The problem is that Democrat primaries, while they tend to produce "moderates", milquetoast rolling-over do-nothings, they fail to look at the serious numbers of voters who didn't vote for them and think "whelp, I guess I can ignore the clear demand for progress" and actually count on the people who they ignore.
The problem here is that you cannot, MUST not forever ignore progressives. Give them seats at the table and committees and govern in good faith and chances to show their mettle, or lose elections and we all suffer.
At some point, in certain dilemmas, the solution for everyone, the quest that we put forward for the righteous and honorable, is that when there is food, zero sum, and someone chooses violence over lottery, the food is to be
spoiled.
Then, have it made clear by one (but not all) of the conspirators to the right thing, that this is what will happen if people do not play nice: the violence will be done and everyone will still die.
"Do not deal in bad faith, excluding wantonly people's ability to participate when you need their support".
When some significant portion of the electorate wants a candidate enough that them running independently guarantees a clear loss for their cause, and then running in the primary means they will lose but maybe not challenge the outcome of the final election, you let them have as big a swing as people might want, and then when they lose, you clap them on the back and say "well, your platform got 30% of the party support, so your platform should underpin 30% of our agenda", and do that.
Not doing that makes you seem like a fucking heel. It made Hillary seem like a fucking heel. So what if Trump is also a heel! The people voting in this, the wrestling fans and the incels and whatnot, those folks,
when they see two heels, they root for the bigger bastard.