The thing is, I'm actually not expecting a 100% shared library, which is another reason as to why I'm imagining the console releasing in fall 2017. Even if the hardware side of it could be finished earlier I think there will be games for each that are exclusive to one of them. Some, maybe a lot, will be cross-compatible, but are we really thinking developers will be forced to develop for all NX-machines? Even if it turns out to be a super easy process I think it's a stretch, and I could see a developer like Nintendo going "this is a better fit for handheld" or vice versa for some games.
So I simply think they'll need some time to get the software ready, and to allow third-parties to have software ready, and to make logistics a bit easier, and be able to sell it as a new exciting product during the holidays.
Now before you go "but they can't possibly keep Wii U going through 2017?!", well...no. And they won't really support it much either. It will sell poorly, few games will come out. But Nintendo themselves will be fine; smartphone games, amiibo and the NX portable will keep the business profitable, while Wii U-owners will be waiting to move on to the next thing in the fall. Despite this, it's still just a 5 year generation for Wii U, it's not in any way a long time, even considering its sales failures. It's 1-2 years below what's usual these days for a console's life span.