Joeseph Goebbels understood that effective propaganda must be first and primarily entertaining. This is why Star Trek was able to get away with genderbending characters like Dax and nobody even noticed. The leftist writers know their agenda, but not how to make it entertaining because the agenda is the story to them. They don’t even have any fresh ideas on the agenda. It’s like branding. You can’t change a brand, like the golden arches, because then people might not realize it’s the brand. So they end up with pukish movies no one wants to see.
I like what you wrote but go a step further:
The leftists who wrote Star Trek abused their freedom to push their agenda.
But now that leftism has prevailed in the media it has established an orthodoxy that Must Not Be Defied. And thus the leftists themselves have created the stagnation of ideas that you so adroitly observe.
#4 DAX always gave me the creeps. Having an affair with Worf the klingon, an affair with a woman, several guy aliens. He/she/it did not care as anything goes went this Trill.