As I said on the previous page, I think the next console will come soon enough that there's no time for an entire new Pokémon generation on the 3DS. However, not soon enough that one Pokémon game alone could keep fans happy through the entire waiting time. What they do now is extending the longevity of ORAS, so that the waiting period after Z won't be too long. It's better to have ORAS -> 1.5 years -> Z -> 1.5 years -> Gen VII; than ORAS -> 1 year -> Z -> 2 years -> Gen VII, after all.
Also, Pokémon has never been a launch title for new consoles. It might be that the NX will come out shortly after Z, and that its first Pokémon games will be out after a year or so. I don't think Game Freak are obliged to have a new Pokémon game ready for launch day. And even if so, it would make even less sense to postpone Z.