I think Nintendo decided in 2014 to ditch the Wii U. By then they knew full well it wasn't going to be the success they wanted it to be.
Iwata even said in 2014 that they were working on a new definition of gaming and it would take two years to realize that goal (2016). They also signed a deal with Sharp for free-form LCD displays in late 2014, displays that wouldn't be available until ... 2016.
We now know due to patent leaks that those free form LCDs are for a game controller/or portable, not for Quality of Life.
NX has been a go for 2016 since 2014 IMO. Nintendo IMO shifted development of Animal Crossing (the REAL AC), the next 3D Mario, and several other 2016/17 Nintendo titles to NX a long time ago IMO.
Development kits have already been give out to third parties for a while, which means Nintendo's own internal studios must've had kits for several months prior to even that.
