I'm not sure why you're all assuming the NX is the Wii U's successor-- they've given no indication whatsoever of what it is.
Nintendo actually have given some indication of what NX is, and they indicate that it's going to be something that spans across different types of hardware, rather than being focused on one specific gaming box or handheld. It could be a new OS or environment that is compatible with home console, handheld, and mobile device - something like Playstation Mobile, or maybe something more like iOS or Windows 10. The hardware updates for the "New" 3DS suggest to me that it was meant to support some newer operating system and software gaming platform. Why would Nintendo update the hardware so much for only a couple of games? The NX as a software/network infrastructure might possibly run on Wii U, New 3DS, mobile, and some newer, unannounced hardware. Think of the Playstation Store spread across PSP, PS Vita, PS3, and PS4. And now that those devices are all supported by Unity (except PSP), cross platform development and sales have become much easier. Windows 10 is getting there too. Soon it will run on phones, Xbox One, tablets, and PC's, all linked to one app market, with Xbox Live integration. Still, this type of NX future seems almost impossible, when you envision it coming from a company that can't even figure out cross game chat, while Microsoft has had the feature for 12 years now.