Here's the argument for Pokemon releasing in 2019:
- Game Freak gives each generation three years of development, with the pre-production/concept time included in that. New Pokemon generally take about
3 - 6 months to design.
- This point was reiterated by Masuda to Game Informer last summer when they asked when we could expect to see Pokemon on Switch.*
- Game Freak has split itself into an A team and a B team. The A team focuses on new generations (BW, XY, SM), the B team works on the rest (HGSS, B2W2, ORAS, USUM).
- DP (2006) -> BW (2010) -> XY (2013) -> SM (2016) -> ???
Because they're fairly candid in interviews and in blog posts, we know what Game Freak has been up to throughout the 3DS' lifespan. While I can't find the post any longer, Masuda once shared on his personal blog that XY began development in June 2010. We also know that SM was in a concept phase before the release of XY because they left the Strange Souvenir for us to find. Masuda passed the torch to Ohmori, and tasked him with directing the Hoenn remakes in 2012; Ohmori then went on to complete SM. Development of SM came down
to the wire. They've also said USUM only had about a year of development time, presumably taking place from Summer '16 to '17.
Game Freak isn't very large compared to other companies working on franchises this size--deliberately, too, Masuda is against expanding too much*. For Gen VIII to be ready for release this year, they must've been working on it concurrently alongside SM and USUM. Just where did they find the time to split their team (and new lead director, Ohmori) over three projects, one of which was their first HD title, on a device with a significant leap in power? It's not impossible, of course, but it's easier to imagine that they stick to the schedule they've been following for the last decade. It takes time to plan out and design big RPG worlds with over a hundred unique Pokemon and character designs, with totally unique assets between each generation. For SM, they claimed they made a brand new engine distinct from the one made for XY, and I'm sure they would need to do it again for a game much bigger in scope.
There's also the fact that Pokemon isn't a normal franchise. The games, anime, competitive circuit, and movies all have to work in tandem with each other. In USUM, they added a brand new mythical Pokemon. Which means we can expect a new movie starring with the Pokemon, and a continuation of USUM's promotion to go alongside of it. Before they can formally announce a new title with new Pokemon, Zeraora has to be addressed.
Again, it's not impossible, but it'd be surprising if GF managed to pull this off.
*Don't have a time stamp for these, but they were mentioned by GI in their
podcast episode about their trip to their HQ.
(Edit: some typos.)