I don't know how long Ps4 and One will last, but I think next Nintendo home console will arrive just mid-gen in a few years (2016?).
This would bring a very interesting twist, if ever Nintendo will prove being still relevant in the home console market.
Who would buy that console? Wii U owners? There's only 3.5 million of them out there right now and they might not be delighted by a short console lifespan. People who didn't buy a Wii U? How would the next console appeal to them if the Wii U doesn't?