So, what, in this scenario, there is no TWOK or any movies at all besides TMP? I dunno, if it didn't happen in the 80s, then either the 90s or early 2000s would see a Trek revival of some kind, whether it be TNG or new attempt at a movie reboot of TOS.
Actually, just for shits and giggles, let's assume TNG still did premiere in 1987 pretty much exactly as is, only there had been no movies since TMP. Off the top of my head, they'd have to created 100% new sets, as I doubt the TMP sets would have been held in storage since 1979 if there had been no other activity in the franchise in the interim. Likewise, aside from the refit Enterprise, there were no ship models from the movies to use, so if the refit Enterprise model were still around, it would likely be used as the other Starfleet ship the Enterprise D meets up with. Hmm, assuming the rest of the franchise still occurs aside from the TOS movies (which I'm assuming we don't even get TFF or TUC in this scenario) let's look at some points posited in the OP:
Ponder how much of current Trek calls back to, or relates to Khan.
Khan still exists, as he was introduced in Space Seed. I imagine if the franchise still wanted to make a boogeyman out of genetic enhancements, they could still reference him. Just instead of doing a Ryan George impression of saying "He's from The Wrath of Khan!" when they hear him, references to him would just be an easter egg for TOS fans.
Ponder the absence of Carol Marcus, David Marcus, and Saavik from the franchise.
Carol Marcus is the only one of those who has been used in the franchise outside of the TOS movies. So we'd lose Janeway mentioning her in The Omega Directive and Alice Eve either wouldn't be in STID or would be playing another character, Janet Wallace, maybe? Okay, writing this sentence makes me realize STID would be a very different movie with no TWOK around, but anyway.
Ponder the absence of the monster maroons.
Then in the TNG episodes that featured them we'd see instead the TMP uniforms or if they weren't available, they just stick with the TNG S1 uniforms.
Ponder the absence of the Kobayashi Maru test.
Was never referenced in the franchise again until the Abrams movies, so very minimal impact. Besides, Trek XI, we wouldn't have an episode of Disco or Prodigy with no Kobayashi Maru.
Ponder the absence of the katra.
Isn't really a major component of the franchise anyway. Besides TWOK and TSFS, the only other times the katra has been featured was in Enterprise's Vulcan reformation arc, and in Disco with Michael carrying a part of Sarek's katra. And in both instances, if there had been nothing previously established, I'm sure something similar would have been thought up.
Ponder the absence of Genesis
The franchise never referenced it again anyway.
Ponder the absence of the Bird of Prey class of ship!
Then they'd just reuse footage of the Klingon battlecruisers from TMP, which several early TNG episodes did anyway. Eventually a new model would have been built, just with no BoP and its pre-existing stock footage, they'd have to use the new ship more often.
Ponder the way that post TWOK, Trek went from being a show about the exploration of the human condition to largely a pew-pew franchise, especially in movie theaters.
I mean, that's not that hard to imagine. Action sells, and TOS itself was sold as an action show. Factor in the negativity TMP received in the theatres, and it's not hard to imagine any future endeavors in the franchise would be more action oriented.
Ponder what we might have gotten instead, in terms of a movie franchise.
Already did that at the start of my post.
