Ask Lemmygrad

I'm not saying Democrats run fair primaries. That article makes the point that even if you run and win as a third party, your new party won't be able to hold members to a party line. If it's already near-impossible to win as a third party and doing so leaves you with such a huge problem anyway, it's not worth it.

If you wanna do democratic entryism, you gotta have an actual threat for if they screw you

I think the appropriate threat (at least right now) for Dems jobbing a primary is for the entryist candidate to not endorse the winner or encourage their supporters to vote for the winner. A "if you screw over my candidate, I won't vote for you" approach. People already withhold votes in the general for just not liking the primary winner enough.

Just like you need a threat to get a hostile party apparatus to take you seriously, you need to compromise some to actually win mass support. "Give me exactly what I'll try to spike your chances in the general" is a good place to compromise, because that wins few people over and probably crystallizes some of your opposition.