The failure of classic Trek wasn't oversaturation. It was creative fatigue resulting in bad writing. Get an independent fresh team and Trek can succeed on TV and in cinemas simultaneously.
The ratings and movie profits don't back you up. If you track both box office earnings and TV ratings, there's a pretty steady decline the more Trek there is available in theaters or on TV.(with some brief exceptions like during a show's first season or for ST: FC.)
(Also, I'm not sure if you're a DS9 fan or not, but that was a very well-written show that never came close to TNG-ratings after season 1)
But it's also true that by the time of DS9 and VOY there was a different business model for TV and a lot more sci-fi shows available.
Well let's see, you first tossed out the success of FC when that is considered the best TNG movie. And then, let's look at DS9 this way. During the 7 years DS9 was on TV there were 14 seasons of TV Trek and 3 movies. That's the highest density Trek has ever had, and yet VOY kept going and ENT was created. Then in the 6 years that lead to Trek being "over-saturated" there were 6 seasons on TV and 1 movie. So was it the amount of Trek that killed it? Or maybe the quality of those final years had something to do with it?
I didn't "toss out" FC, I admitted it was an exception, and it was a good movie.
As to your other points, yes, Ent and NEM had low ratings during a period of "low saturation," but IMHO the "high saturation" had already had an effect on the Trek franchise by that point.
Look, I'm not saying that the quality of writing doesn't contribute, but I'll leave you with this:
STIV was the second-biggest hit at theaters of all the Star Trek movies. There was no Trek on TV when it was released. STXI was the biggest hit in theaters by far, again when there was no Trek on TV.
I don't think the success of STXI was because it was such a great movie, I think it was because there hadn't been a new Star Trek movie or show for a while, and there hadn't been new TOS-era Star Trek in a LONG while. So I think pent-up demand had a big role in its success.