My reasoning for this:
1: This new game is most likely likely a new generation, since whenever mainline Pokemon jumps to a brand new console, it starts off a brand-new generation ( Ruby/Sapphire on GBA, Diamond/Pearl on DS, X/Y on 3DS). Also, the last two generations before Sun/Moon have only consisted of two games each, so to me it looks like Ultra Sun/Ultra Moon are the final games of generation 7.
2: And since this mainline Pokemon on Switch will most likely start a new generation, there is no way a new generation would start only two years before the previous one did. The shortest generations in North America were Gen 1 (2 years) and gen 5 (2.5 years). Generation 5 was this short because the 6th generation transitioned to a world-wide release instead of Japan-first, and the gap in Japan was still 3 years (fall 2010 to fall 2013). Had this transition not happened, Gen 5 would be 3 years long in NA (spring 2011-spring 2014). And Gen 2 came early for NA becaause they were playing some catch-up with Japan.
3: With better hardware comes a bigger and more ambitious pokemon game, which I think they won't be able to finish it by fall 2018 even if they wanted to, since Game Freak is a quite a small studio. So the game will naturally slide into the fall 2019 release window. The last few Pokemon's have all released in October/November, so there is no reason not to believe this won't happen this time around. I think Nintendo might even hold the game until fall if its finished in the spring or summer to make it the premier holiday title for the Switch that year.
So yeah, I'm basically saying 2018 will be a Pokemon-less year, which I don't want to happen myself, but my thinking says otherwise.
Bet with Intrinsic:
The Switch will outsell 3DS (based on VGchartz numbers), according to me, while Intrinsic thinks the opposite will hold true. One month avatar control for the loser's avatar.