I love my 3DS, and will never get rid of it. It has a fantastic library and the glasses-free 3D effect is really cool. That said, I still think the new Pokemon titles should have also come out on Switch. After all, it has a much better screen, is way more powerful and it's new. The latter may be a silly thing, but since it's a shiny new gadget I'd rather use mine than my 3DS for now (of course, I'll gladly come back to it in the future to continue the Ace Attorney saga and many other games I've yet to play or finish).
Besides, gen 7 hasn't made any good use of the 3D effect or touch screen (if at all), which are the two main features of the 3DS. At this point, there's nothing to be missed in a Switch port of UltraSun and UltraMoon (as opposed to gen 6, which had the excellent PSS and DexNav features).