Only that it's uncommon - not unheard of, but uncommon - for the same Legendaries from past generations to be in two games in a row. At least these days. In earlier generations it wasn't quite so unheard of, but it hasn't happened recently. Although Cresselia and Darkrai would make a certain thematic sense for USUM and I was somewhat surprised they weren't in SM...although Lunatone and Solrock weren't in SM either, so I suppose fitting the theme didn't mean much in those games.
I wouldn't count on that if I were you. If Nintendo are serious about supporting the 3DS beyond 2018 - which, according to Reggie, they are - we are almost certainly going to see more Pokemon titles appear on it and, if they want to start a new generation on the Switch, it would make sense for them to confine remakes to the 3DS. Well, it would depend on their plans for the 3DS' life going forward and if there will be a successor I suppose, but it's not outside the realm of possibilities that we would see Sinnoh remakes on the 3DS instead of the Switch. Although given that Nintendo are a pack of liars when it comes to supporting their consoles I could understand your skepticism there at the least, even if I don't particularly want to share in it...