There are still well over 100M people that are yet to buy a current generation console, probably somewhere around 150-200M, based on the install base last generation and potential for growth into younger gamers or people that just never tried out gaming before, even taken into consideration that many Wii owners had no intention of gaming on a console again.
If NX is a new unified approach as it's been rumored too be and then that platform goes on to get all of the same 3rd party titles, that some people who just never buy a console because they're never home, but also never buy a handheld because it doesn't have those blockbuster games that they may want to play, then there's definitely no issues in Nintendo tapping into that area of the market.
TBH I think NX is going to be the end of this wall between generations for Nintendo, since API and OS have become more flexible and companies like Apple can transcend generations there's little reason to keep the walls up, them joining Khronos is quite telling really.
PC is fine having constant hardware upgrades, Nintendo wouldn't be going to the elaborate degree that PC does, but more likely release an updated model every 2 or so years, that way Nintendo can just incorporate better visuals and some extra features into the newer systems, but most games would still run on anything within the NX family.
Thinking that the market is all in the bag because your friends all play PS4 is ludicrous and this is coming from a PS4 gamer. TBH I'm personally holding off on buying a Wii U, partly because I don't want to buy one and then find out that NX will play it's games, along with being this unified machine. Also I'm partly on the fence because Sony has so many games coming and I see no point playing 3rd party anywhere else.
I would be fine getting NX if Nintendo can unify their development, as this would mean they aren't devoting separate sets of resources to handheld and console, but rather one set for everything they used to make and everything will run on both pieces of hardware, then Nintendo can devote some of their resources, like a good 30%+ to making new IPs or actually making more use of their other IPs that get ignored because Nintendo likes to play it safe using Mario and co so much.
Hell a Nintendo that does this, making games that appeal to the western market or a wider, diverse range of customers all over the globe would likely have no issues in getting 3rd party on board, because they'll have that install base gaming on their platform once again.
Releasing at the end of 2016 is not an issue provided that Nintendo has games suited to their audience, they have a clear marketing message, that describes a platform people want.