I think the only time franchise fatigue sets in is when thing start to drop in quality. The only reason franchise fatigue set in with Star Trek is because people didn't like the later stuff, like Voyager, Enterprise, Insurrection and Nemesis as much as the earlier stuff. If the stuff coming coming out continued to be as good as TNG, DS9, and First Contact, I don't think people would have minded having so much Trek.
Yeah, the post-TOS
Trek ratings drop started with DS9.
One other thing, which is that the standard for people not minding so much
Trek wasn't that it be as good as TNG. It was that it be engaging to the general audience, period. Post-TNG TV Trek wasn't engaging to the general audience, in every instance, even in the case of DS9. As advanced as DS9's writing was
for Star Trek, it was still behind the curve when it came to other TV drama out there.
Hill Street Blues already had DS9 in the rear view mirror a decade before "Emissary" aired.
I make this point, because being as good as what has come before in a franchise is just treading water. You don't want to tread water. Every new entry has to keep upping the game, to keep pace with the times. People are starting to get tired of MCU films, because they're starting to look too cookie-cutter. Arguably one thing that hurt post-TNG TV
Trek was succumbing to the temptation to stay too close to cookie-cutter mode.
If the new SW films and other spin-offs, e.g.
Rebels and
Rogue One, can keep innovating and just plain being good, period, then people won't tire of them, ever.