The problem with USUM is not that it's 3DS-exclusive- yeah, it sucks for me personally, but the business case for keeping it to the 3DS is an understandable one.
The real problem is Game Freak is dragging its feet and failing to be on the new hardware before 2019. I think it's probable that waiting so long to get onto the 3DS damaged X and Y's performance, particularly in Japan. Of course, it's even worse this time than it was last- Black and White 2 could be played on 3DS, and even had 3DS-specific downloadable software available to complement it, and we've heard that a good portion of the owners of that software played it on their 3DS. This time, there's going to be no way to play any kind of Pokémon RPG on the Switch for years to come, which cannot be a positive for the brand.
This is, of course, assuming that we're not seeing the end of Pokémon generations on dedicated hardware, and that Game Freak aren't moving them to mobile to chase that Go money.