Given both iOS and Android support hardware controllers... and given how closely Nintendo ties in their games with their controllers... and given that you can get good margins off of hardware, a Nintendo-branded controller for mobile devices, that is required (or highly recommended) might give them the best of both worlds; they could continue selling hardware, continue controlling how the games get played, and still release them for mobile devices.

And given how consoles generally (start) selling at small margins, they might be able to make more money off of selling controllers at a higher markup—especially if they sell people a new one every two years (when they upgrade their phone), rather than every 6+ years (when a new console comes out).