I think the modern writers have forgotten that some of those gimmick episodes of TOS happened because they had a shoestring budget and had to reuse whatever cowboy or city set was available.
Ahh so it's the reliance on gimmick episodes that you think is the real problem, not just the frequency of them. If that's the case, I'd have to actually go back and check ENT and VOY for this, because serialized episodes tend to rely on some kind of gimmick, especially in Star Trek where we often need a sci-fi whatzit to create an issue for the crew. I actually am curious if VOY had more gimmick episodes than ENT then, since it was far less interested in episodic storytelling overall, even if they did apply it sometimes. But then again, ENT was more reliant on "Star Trek" as an IP to exist, which got way more fanservicey than even the contrived Romulan, Klingon, and Ferengi episodes in VOY (but then "Regeneration," is one of its stronger episodes for exactly that reason).