IIRC, plans for a Star Trek show came first, not a feature film. When SW came out and became a smash, the pilot's script was turned into TMP. Had Trek gone down the route of TV show instead of film, there's a decent chance that it would have failed, like Battlestar Galactica and Buck Rogers at the time.
Besides, prior to that, there wasn't much demand for sci-fi movies anyway. 2001 was more of a critical favorite than a financial smash.
You are incorrect. PLANET OF THE TITANS had Phil Kaufmann as director, Jordan Belson for optical effects, Ken Adam & Derek MEddings (both of Bond fame) for production design and miniatures, and was basically a 'go' project even before having a full script. It was cancelled within a month of SW coming out, and phase 2 arose out of those ashes for a short time.
As for the demand for TREK SF ... you had to be there. the mid 70s had an insane demand for trek (30,000 at a chicago con) and any action adventure film (even of mediocre quality) that came out ahead of SW would have cleaned up like you wouldn't believe.
Yes and considering the Film would have ended with Kirk and company pretty much stuck in the distant past founding the advanced civilization they were looking for in th film Star Trek probably would have ended then.
There were endings where they were in the past, endings where they were in the future, and probably a security ending where a few of them stayed behind and the rest went back to century 23.
But I'd've been fine with them ending it then, too, though I doubt it'd have kept sequels from happening (look at the APES movies.)
I'd also have been fine with seeing them do TITANS as the new movie, with Capt April or whoever, as a standalone story.