I think Nintendo is releasing the NX way too early. The PS4 and XBox One are three years old, and will be selling for at least three years more. This generation is far from over. People with a PS4 or a Xbox won't be buying a NX as it won't be a mayor jump from the current consoles.
And they shouldn't be porting many Wii U games, so the Wii U has a legacy.
No one said that the NX will mark the next generation. While the WiiU was a little early for this generation, the NX will be a little late. Nintendo just doesn't fit in the cycle with Sony and Microsoft anymore, because Nintendo consoles aren't really an alternative to PS and XBox anyway. People buy them either because they are die-hard Nintendo fans or as a secondary platform. I reckon it will be the same for NX, except if Nintendo presents something really revolutionary.