I'd say both over-saturation and quality had a lot to do with the decline early this decade.
The last two TNG movies were considered bad, Enterprise was cancelled, and Voyager was considered lackluster and disappointing (if I'm reading everything correctly).
The last episode of Enterprise was a TNG holodeck recreation that got blasted by members of both casts.
One good thing about the over saturation is that fans get a lot of extras like articles, books and forums--this forum might not be as big or active if Trek stopped at TNG.
I watched Spartacus, and it was only 3 short seasons with only 10 episodes per season for the last two.
It had a forum that was very active and hot while it was on, but once the show ended, the posts fizzled out and now it has only a few die hard fans posting.