Soundwave on 03 September 2014
Both DSi and Game Boy Color had lifespans of about 2 years (give or take) before the 3DS and Game Boy Advance came out.
I'd expect the same here ... new Nintendo fusion portable coming in 2016.
By 2016 a handheld capable of Wii U graphics should be possible, the Tegra K1 Nvidia tablet is already out today and that costs $300 with a really high resolution 8 inch screen.
Give two more years for that type of tech to scale down in cost and Nintendo will likely use a smaller/cheaper screen and you have something probably with some serious horsepower for about $200-$220 IMO.
