Er; question; is that Arakawa, or rather the sentai era during which he's been a main writer?
I'm going to say that I think it's Arakawa. Because even when he was a secondary writer on older shows, he focused more on showcasing comedic villains (his 3 Idiots arc in Dairanger) or he'd just ignore the regular villains altogether (like he does in a majority of his Jetman episodes in favor of dumb stuff like Dryer Jigen).
Are there some post-GogoV sentai villains you've been able to enjoy (besides Jannu and Gileton that you've already mentioned in your post) ?
You know that a lot of the problems I have with modern villains is that I like the actor villains and not the suits. There's still villains I like, though, like Long in Gekiranger and Takeshi Asakura...
Don't you feel that in Agito, the Lords have the same problem the Grongi had in Kuuga?
I think the Grongi needed to be a stronger force because every other human character in Kuuga is unbelievably sunny and nobody has a problem with anybody in that show. Kuuga started the trend of setting aside a regular group of villains in favor of monster of the week types, but I think Agito made up for doing something similar with the Unknown by at least having other obstacles for the heroes, like antagonists on the human side. There's nobody like Houjou or Kino or troublemakers from the Akatsuki in Kuuga. (Agito also made good use out of being the first series with more than one Rider, having them spend half the series as misunderstood opponents.) So I thought that helped make things more interesting in Agito. Subsequent shows tried their damnedest to always recreate Agito, but were lazy about it and didn't have the intricate planning Agito did, so they take that concept and fumble it and you get annoyances like Kusaka or Mutsuki, who are shoehorned into their shows and are just devices and not written in in any natural or rational way.
It's obvious Kuuga's whole thing was taking a realistic approach to kaijin attacks and how it effects the community, so you're not going to get scenes where they're like "Mwa-ha-ha, soon Tokyo will be ours," but that doesn't mean you just shove the villains into the background. (Although I always thought for how realistic the show wanted to be, they needed to hire
way better actors for the Grongi instead of the hams and ex-wrestlers they did.) The Grongi are preferable to, like, the Worms or Undead IMO, but they really stick out as not meeting their potential, not having their mysteries solved, and I just think it's because of how Arakawa doesn't seem to care about writing villains.