The games exist because Gamefreak felt there was enough to add to what was already there. Here is a good interview explaining their mindset during development:
http://nintendoeverything.com/game-...ultra-moon-development-new-features-and-more/
I honestly think the game is gonna surprise a lot of you in terms of what's added and how much the story is going to change. I personally would not have minded sequels, but I can also see that with SM having such a complete story and development arcs for its characters, its a cool idea to revisit then while also adding a whole slew of new things for added depth.
Another thing is, that USUM is dealing with a lot of issues that plagued SM, and the games honestly deserve the refinement they are being given here then say what we got with Emerald and Platinum. More interactive Pokerides, Trials with some much needed depth and presentation, its all those little things that I can justify a purchase.
Lastly, I cannot find the other interview, but its clear that Gamefreak does not intend to release Pokemon on Switch until they feel they need to. Said interview has a blurb that has em in record saying that (paraphrasing here)they felt a one year gap happened this time around because of all the ideas that they have and using them right now, but also because two or even three years later Alola won't be so fresh and recent in everyone's mind to justify a purchase. It sounds to me from all their interviews recently that they wanted to do one last push on the 3DS before focusing on the Switch, basically leaving 2018 as a free year to do what they have to do.