Nintendo can support Wii U by 2022 or more and Wii U's hardware will be used in next Nintendo's console as Nintendo confirmed also I doubt that even a quarter of console gamer's will have 4K TV's by 2020.
Wii U's capabilities are decent, look at Super Smash Bros U and you can see that Nintendo can make a 2D and 3D Mario running at 1080P60FPS and other games if they want them to be 1080P60FPS like Luigi's Mansion or F-Zero could be on Wii U. If Capcom was to make an exclusive Monster Hunter game for Wii U only then it would allow considerable improvements in visuals while also run at 1080P60FPS or just aim at 30FPS and double the details on the screen if they want too.
Soul Calibur, Tekken, Dead Or Alive, Virtua Fighter and Street Fighter could run on Wii U at 1080P60FPS with modest/noticeable improvements in detail over HD twins counterparts.
I am sure Nintendo will release a new handheld by 2018 and it would be wise if it used Wii U's architecture in case 20-16nm node is ready for use and mass production of chip which would allow them to make a SoC of Wii U's hardware(CPU, GPU, Memory controllers, etc...) which would allow them to save some silicon and shrinking it for 20-16nm could make chip around 70 mm^2 with 3DS/DS hardware chips in same SOC.
They could balance it, if they use 720P/HD screen then they could cut eDRAM from 32MB to 16MB since they don't need that much bandwidth nor it has 1080P screen and would save 10 mm^2 so chip would be below 70 mm^2.
If Nintendo was to put somewhat downscaled Wii U's hardware in their next handheld then it would be an ease to produce games for Wii U and allow cross platform play without any issues...







