TMP is a film that benefits from repeat viewings.
When most people saw it for the first time in 1979 the main thought was "visually impressive...but long and slow" and people weren't going to go back to the theater 3 or 4 times to rewatch and notice the quality of it.
And since the VCR was a brand new technology most people didn't have that option either.
Today with the ability to watch it any time you can go back and appreciate the other things in the film and get over the fact is was slow because you already understand and accept that.
Ironically I think TUC is the opposite. I think it's a film that was pretty impressive when you saw it once or twice. After repeated viewings though you notice a lot of things that, at least IMHO, make it less of a great film that it seemed to be after you first saw it.
And, even though I don't personally agree, if you think Bennett and Meyer started making the films look cheaper, I don't think you can hold them responsible. They really were working on a very limited budget, especially for a sci-fi film in the 80's.
After Roddenberry spent money like a drunken gambler on TMP Paramount was absolutely determined to not let that happen again and basically told Meyer and Bennett.....this is all were giving you, make it work and don't ask for anymore and I personally think they did a great job with the pretty small amount of $ they were given. Especially when compared to "The Empire Strikes Back" and "Return of the Jedi" which had blank checkbooks because Lucas and Fox knew 100% that the films would make back far more than whatever they spent.
Star Trek films never had that luxury and basically everyone held their breath when a new film came out hoping it would be a success.
To be fair it wasn't Roddenberry spending like a drunken gambler, it was the studio....the film started as a pilot for a tv series, and then a tv movie, and eventually a feature film once Close Encounters did decent money. Sets were built, and rebuilt, and models made each time. Then they tried basically setting up an effects house, which then didn't finish anything, and because they had presold the movie, including TV rights, for large sums, an would lose large sums in litigation if they didn't get it done in time, they had to keep throwing money at it to get it done in time.
It's like a 6 year project that went down to the line, with it being basically a finished draft at the premier. Not even a final one. So it cost a lot, but not really because of gene.
Star Trek V has almost the same problem, but inverse with the money (the studio kept cutting budget)
If it's one thing those films should have taught the studio, it's to be more flexible with release dates and pay attention to things like strikes. Your audience will understand those delays more than a rush job movie. (I know....how does a 6 year cycle constitute rush job? 500 effects shots in six months. That and more false starts than the Olympics.)
The later films are not done one the cheap because these studio didn't make money on the first either....they just wanted to maximise profit rather than have artisan work like the first. Hire cheaper crew and composers, recycle effects and models, and even rewrite the concepts to draw in a perceived wider demographic. The only TOS film to have an ambition beyond this was STV....which ultimately got battered by two strikes and large budget cuts when shooting began. One can't help but feel Shatner didn't make the right friends at the studio.
The films that my friends and I felt were the best as children, really, in retrospect, weren't. Khan took the world building back by almost a century, and TUC took the character development and social science fiction back by almost the same amount, for the sake of clumsy polemic.
But TMP? Genuine motion picture, genuinely doing something new and progressive.