IAMD1&2 are the best of the M,M sequels, and near the top of ENT episodes, but they're nowhere nearly as good as M,M. One of the things that dragged them down was all of the continuity porn: inventing the agony booth, let's squeeze in a Gorn (but not bother to make it an actual Gorn), etc. By cutting out all of the cruft and focusing only on the actual story, they could have made a very exciting one-parter instead.
That said, "Mirror, Mirror" should have really been a one-off, in which events were precipitated by a momentary alignment of two otherwise divergent universes. There's no intrinsic reason why the pasts and futures of the two universes should stay in alignment. The fact that TPTB kept going back to the same well was a clear sign that they were running out of ideas.
The "franchise fatigue" that audiences experienced in terms of Star Trek wasn't really getting tired of the franchise per se. Rather, it was in seeing the same basic things over and over, essentially just recycled. Navel-gazing, by which I mean focusing on things that only hard-core fans cared about, is an example of that, as is too many Mirror Universe episodes. "Franchise fatigue" is a self-serving myth invented by Berman to cover for the fact that TPTB couldn't come up with anything original any more. Audiences got bored. I don't blame them.