Right now it seems Pokemon Company/GameFreak (they are separate from Nintendo) is taking the 'wait and see approach'.
I thought I remember the same reasoning for them making B&W 2 for the DS rather than for the 3DS. Which is because the userbase on the DS was a lot larger than the 3DS at the time. Now the same goes for the Switch where the 3DS has a huge lead in player base compared to the Switch, and likely wouldn't help with the supply constraints the Switch seems to have right now.
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edit: found the quote
The primary goal is to get as many people as possible able to play the game. With the Nintendo DS, it's the handheld hardware system that the most people in the world have. Those who played Black and White may not have a 3DS, but they definitely played on a regular DS.
http://www.nintendolife.com/news/2012/09/ever_wonder_why_pokemon_black_and_white_2_isnt_on_the_3ds
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I do believe 'Stars' morphed into US & UM as some people will likely still buy both versions rather than one (ala Platinum, Emerald, etc.). The next generation of Pokemon likely will be released on the Switch in say a year or two after the supply issues disappears and the userbase grows. IMO there will be no future 'handheld' dedicated system like the past for Nintendo.
Now I see why Nintendo had solid projections for the 3DS still for 2017, because of this series of games.
