I disagree with the sentiment expressed in the article.
To me, what made Trek great was that it was more than TV show: it was a vision.
It was idealistic, yes, but it wasn't pointlessly idealistic: it was giving us a grand dream of a bright future.
Just listen to Dr Michio Kaku talk about future tech or space and invariably reference Trek, listen to his enthusiasm.
That's what made TOS & TNG so great: they did extreme (especially TNG) idealism right; which is extremely rare in entertainment.
Comparatively, it seems to be easier to make quality "dark & edgy" works than idealistic ones.
DS9 seemed to be about undermining all of that. Yes, some episodes were very well made (The Visitor, In the Pale Moonlight, Way of the Warrior*, Inter Arma enim Silent Leges,...), but the near-constant undertones either undermining Trek's pre-established ethos or pushing religiosity, it seemed to be trying very hard to be the "anti-Trek" Trek series.
And in doing so, it became "just a show". Entertaining, but nothing more. Certainly not inspirational, unlike the prior series.
And let's not forget that, while DS9 wasn't the worst-written series overall (TNG>TOS>DS9>ENT>VOY, IMO), it gave us the worst Trek episode, by far: The Reckoning. The Reckoning makes VOY: Threshold look good by comparison.
*note: quite an underrated ep, that.