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.
The thing is, TOS and TNG both relied on alien cultures from all over the universe that were anything but idealized to create conflict and show contrast. What DS9 did was reveal what it's like for our idealized humans to live with these non-idealized aliens throughout the series. While the Enterprise was able to warp away each week, DS9 stayed put. Cardassians, Bajorans, Klingons, Romulans, all were created on TNG/TOS. On DS9 their problems could not be always resolved at the end of each episode. They showed us what it was like to live day to day deep inside the imperfect (yes imperfect) universe envisioned on the previous Trek shows. And one of those imperfect races closeby happened to be deeply spiritual. Yes, Star Trek has shown that humanity is not the only species which has religious people among them. But DS9 faces it and does not warp away from it. That's not "pushing religiosity" (unless you're a theophobe I suppose) -- it's facing and respecting other cultures. Very "Trek" if you ask me. So I'm sorry, but DS9 is not "just a show."