I'd like to imagine since the Sonic Mania trailer says it's Spring 2017, it'll be available for the NX launch as well. Since it's being done by a different team then Project Sonic maybe they thought they couldn't reveal it's an NX game since Nintendo hasn't announced the system yet while the Project Sonic team didn't care? I mean it's a retro 2D platformer staring Sonic, two things that sell the best by far on Nintendo platforms so Sega wouldn't be that stupid not to release it on the NX, especially at launch when hype will be highest?
Then again this is the same company that hired the developers of Pokemon to make a 2D Donkey Kong County platformer staring a cartoon elephant and then released it on every platform, except Nintendo ones, you know the platforms with the majority of the audience for said title. And of course said game sells like **** since you know, the audience for it is an Nintendo platforms which Sega refused to release it on.
So I'd like to remain optimistic that Sonic Mania will also be hitting the NX as well but considering Sega's long continued history of dumb decisions, I'll keep my fingers crossed until I see the official announcement.