The 3DS is more like to get replaced. If the dev kits for the NX were actually released in October/November (at it's earliest, anything earlier than that and we would've gotten some proper leaks by now), and if the NX is a home console, we're not looking at a 2016 release. Even the Wii U's dev kits were released 1.5 years before its launch. And we all know the quality of its launch line-up (sloppy ports of old multiplat games and only 3 or 4 new games for a good while).
The 3DS is pretty succesful and that's why Nintendo needs to replace that system. It sounds counter-intuitive, but the sales of the 3DS are winding down, they cannot let their most succesful platform fade into nothingness. The 3DS had its time, they can do way better in terms of specs for a handheld. The Wii U on the other hand, it's still selling the same as it did two years ago. Third party support isn't magically coming back either, so there's no hurry with replacing that system. They almost have to start building their userbase from zero anyway.
edit: Also, the idea of launching a new home console in the year the PS4 will be in its prime.. They're going to set themselves up for another failure if that's what they're planning to do.
