I'd be more inclined to blame TPC than Game Freak, at least to a point - it's difficult to really say without knowing what goes on behind the scenes, but looking at what we do know, it comes down to their deadline vs. Masuda's work ethic. At what point does their deadline become too short for them to make a higher quality product, and at what point does Masuda's insistence on smaller teams compromise their ability to do that with the time they have? Because really there was no sound business reason for them to not include every Pokemon in the games from the get-go: the models are the 3DS models, there is no competitive game balancing that justifies a smaller roster, and there is no way the Pokemon included in the games are all thematically suited to the Galar region, so it wasn't a creative decision. It was clearly a compromise, but was it due to a lack of time, a lack of resources, or some combination of both? Masuda's reaction makes me inclined to lay the blame on his shoulders, but what they've done with the DLC and the Pokemon they've added back in is very illuminating as it does suggest time was a major factor, and that'd be down to TPC putting pressure on Game Freak to release new games to tie in with the merchandise, because Nintendo definitely wouldn't apply pressure to have the games out by the holiday season.
With SnS Dexit is definitely only one part of the problem, though - whether you enjoyed the games or not, I don't think there can be any denying that they cut corners in other areas of the games, as you say. You can excuse the narrative if you enjoyed it (after all, there is something to be said for minimalism as a storytelling tool if it is done correctly, your mind fills in the gaps and fandom theory runs wild; it works like a charm for the Zelda franchise to name but one) but there is no way the visuals can be excused. There is a shocking discrepancy between the new move animations and the old ones (people do focus a lot on the old moves, but they neglect to mention how flashy the new moves look, and I think it's important to give credit where credit is due in this instance) and the Wild Area visuals can only be described as poor, even going beyond the N64 trees. Part of this can be excused as Game Freak working on new hardware, but at the same time, the resources they have at their fingertips must be staggering, and they're not using them. Pokemon is definitely a series that should have a huge development team and a colossal budget, and there is no excuse for it to NOT have these things. But with SnS it clearly didn't. That compounds the issue really, as it's not just Dexit that is the problem with SnS, so it's all too easy to write it all off as haters gonna hate. It would be nice if Dexit was only a problem with SnS, but Masuda's comment that this will continue in future games is extremely concerning...honestly, every time Masuda has opened his mouth about SnS it's only been to twist the knife in an already irate part of the fandom.
I think Dexit is still worth discussing because whilst it's unlikely Game Freak read all the comments on the internet, it's always worth voicing your displeasure, and only by doing this can you effect change...and you can definitely effect change by doing this. It's already happened within the confines of SnS, and the backlash could be all that prevents them from doing it in future titles. Given how close-mouthed Game Freak are about their business, you never know.