I have serious reasons to believe Pokemon actually just started development between April and June, and so will be a 2019 title.
https://youtu.be/vB8EskmscKg
In this interview (at 1:28), Masuda states that they don't know what to do with the Switch (regarding Pokémon). He mentions that at first he thought that people were going to use it mostly as a home console, but he was surprised to see people taking it with them, and wasn't quite sure who the main audience of Switch was. He even asks the interviewer how he uses his own Switch, and seems very surprised when he mentions that he always brings it with him on the train.
Unless Masuda is blatantly lying, I think until recently they were undecided on the future of the franchise, but they finally decided to embrace the Switch (or Nintendo just told them they are not planning a 3DS successor)