It's like I always say, look back at the various 90s Trek shows, with 26 episodes a season, there were usually only about 10 good ones anyway. Or, there's the Futurama joke about TOS, 79 episodes, about 30 good ones. Which also works itself out to 10 good episodes a season. Though in the case of season 3, that might be being generous...
I disagree. There are 20 good (or better) episodes in TOS S1 alone. Maybe more.
The City on the Edge of Forever, Balance of Terror, The Corbomite Manuever, Where No Man Has Gone Before, The Menangerie Pts 1 & 2, The Devil in the Dark, This Side of Paradise, A Taste of Armageddon, Arena, The Galileo Seven, Errand of Mercy, The Enemy Within, The Cage, The Return of the Archons, The Naked Time, Shore Leave, Tomorrow Is Yesterday, The Conscience of the King, The Squire of Gothos.
And Charlie X, Dagger of the Mind, and Court Matial are all just fine.
Heck, the best seasons of TNG & DS9 are loaded with good stuff (3-6 on both).
Take TNG S3, where the show started to really thrive. BOBW Pt 1, Yesterday's Enterprise, The Offspring, The Survivors, The Defector, Deja Q, The Enemy, The Most Toys, Sins of the Father, & Sarek is your 10. And entire NuTrek season of home runs or triples.
The good stuff beyond that is all just gravy. Sure, opinions may vary, but I found Who Watches the Watchers, Allegiance, Evolution, The Ensigns of Command, The Hunted, The Bonding, The High Ground, Captain's Holiday, Tin Man, and Transfigurations to all be pretty good or better.
To my way of thinking, that is 2+ seasons of NuTrek material in a single season, all of which is good or better. I will take that every time.
VOY was more hit and miss, but the stronger ENT seasons (3-4) had lots of good stuff. Prodigy easily had 15/20 good ones per season.
Sorry, I just don't buy that generalization of the longer seasons. Now, there were some seasons where what you describe certainly was true. But that has more to to with the show itself than the 26 episode format. Plenty of exceptions.
And with the serialized storytelling common now, there is only 1 story per season. The episodic shows and shows with more episodes had more stories and more development and less plot service. I will take that everyday over a clunker here and there.