(copying off of a post I made in r/startrek, with minor changes to remove the need for context in that discussion)

So this is brief, but I'd actually had a theory recently on it affer watching "Force of Nature" that ultimately, the Excelsior's transwarp drive was a failure. A common critique of the episode is that if the anomalies were growing so quickly - enough to become pronounced within a few decades - why hadn't they happened anywhere yet? Sure, that area of space may have been more susceptible to it than others, but warp drive had been around for millenia (when you take into account ancient species). Why just now, and why so fast?

Well, the warp scale changed. Federation starships were able to go faster because of a new warp tech pioneered on the Excelsior, and could now cruise much faster than they used to - even taking account the afformentiomed scale change.

And when the damage becomes apparent, what does Starfleet do? They order their ships back to Warp 5. What's notable about this is that this was approximately Warp 6 on the old scale (213.75c vs 216c), or the cruising speed of the Constitution class - the fastest class of starship until the Excelsior's drive experiment.

They figured out how to do something with subspace to go faster. But it damaged subspace; and I think it's telling that shortly after this, we actually do see fewer Excelsiors (with the exception of Dominion War battle fleets, which were during a time of an existential threat to the Federation).

So ultimately? The great experiment, and the class itself, were failures, and had to be swept aside for alternative technologies, despite a few decades of seemingly successful service.