Pokémon took a year off in 2014, between Omega Ruby/Alpha Sapphire and Sun/Moon. Pokemon Switch will obviously be the start of a new generation, so I think it wouldn't be too crazy if the franchise took another year in between Gens.
I'd personally be totally fine with a 2019 release.
2018 is looking likelier by the day. There's no more new Pokémon, and the anime's started actively rushing to get through the Sun and Moon story- we're basically halfway through after well less than a year.
I wonder how high a Pokemon Switch attach rate could be, considering Zelda and Mario Kart are crazy high. Definitely will be a drop from 3DS numbers, but depending on how many Switches are out by that time, I could see 10 million being possible.