Well, the typical lifespan for a console is about 5 years. SNES, N64, and Gamecube all had lives that long in America and Europe (SNES had an extra year in Japan). At 4 years, the Wii U's lifespan wouldn't be cut off THAT early. And given the system has received mainline entries from virtually every major Nintendo franchise, bar Animal Crossing and Metroid, I'd say four years, after the release of the new Zelda, is a good place to end it. Especially given its failure to sell all that well. Even Gamecube lasted 5 years, but I believe even the Gamecube has sold better.
If Nintendo can get a console that competes reasonably from a power perspective 3 years into the lifespan of PS4 and One, they might stand a chance of attracting third parties and core gamers outside of the Nintendo crowd. They unfortunately squandered what the Wii did for them with more casually minded gamers, so that seems like the route to go if their going to start rebuilding their market share.
Honestly, the only stuff that would really worry me would be that it would be years before we received new entries into Smash Bros and Zelda on the console. Personally, I'd like to see them port Smash 4 to the NX, with some new modes and characters, rather than starting the whole game from scratch. Produce a proper adventure mode, add a few more fan favorites, bring back the Ice Climbers. Hell, they could potentially make some of this stuff available to the other versions as DLC if need be (aside from Ice Climbers, I guess). I don't really think we NEED another Smash after 4, but the NX will obviously need one. So why not?