Disclaimer: Opinion.
I mentioned in another thread, I don't think the Wii U was EVER meant to be a huge success. I think it was a stopgap and Iwata estimating huge sales numbers in the beginning was either him being completely clueless (since this is Iwata we're talking about, I highly doubt that) or him playing investors like a fiddle (which is very Iwata.) If the rumors of NX as a hybrid are true, that's in line with what I expected.
Handheld has been Nintendo's strength for a long time, to the point that during the Wii years, most of the "real games" were on the DS, not the Wii. It's no secret that maintaining two platforms, especially in the 3DS/WiiU era has been painful for them, and it's not something they ever really wanted to have to do, it's just that tech limited them to need to keep the hardware seperate. That era of tech is coming to a close - there's no reason a console needs to be separate from a handheld in the era of phones, tablets, AppleTV's.
The nature of the Wii U being sort of a handheld system that's also a console (but not really) always seemed to me they were nudging toward. From the moment "Cafe" was rumored that's what it seemed like. I love handheld gaming and love the offscreen features so it always seemed logical but incomplete. And then as soon as WiiU launched Iwata discussed the plans for unifying the handheld and console hardware divisions, building a new HQ to house it all. The merging of all the software units that was rushed through as an emergency action upon his death was not something they could conjure overnight - that was his own plan in the works but wasn't technically supposed to happen yet. The moment WIiU launched everything in Nintendo started moving toward unified hardware.
At his last investors meeting Iwata answered a question about NX (after saying he wouldn't answer questions about NX :D) he commented that he was aware of the difference in how Japan and the West have different play preferences (mobile vs. home console) and they are always trying to figure out how to cater to both with NX. Most signs seem to point to a 1-2 piece hardware that is both portable and TV based with a unified software lineup.
What that means if true is that WiiU was, internally, always intended to last through the HANDHELD cycle, not the console cycle, as a hybrid would have to replace 3DS first as it's naturally at the end of it's console cycle. Meaning this is exactly when it was intended to remain current.
But they'll continue supporting it with software right after release (they still sold Wii for a year or two after Wii U as well. And Pokemon B&W release on DS only after 3DS released.)
Speculation on NX being its own thing, but as it relates to WiiU, my opinion is WiiU was a test project and a holdover to give Nintendo a relevant HD platform while waiting for 3DS's sun to set to release the unified new platform that replaces them both, ending software droughts due to supporting one instead of two platforms. It's also possible WiiU software runs on a handheld battery powered NX. That means Splatoon in the park! NGYES/WOOMY!! WiiU seemed questionably powerful to a lot of console fans, but if WiiU was a precursor for a battery powered WiiU that fits in a pocket, that would be darned impressive.
I'm sure NX will be somewhat different than I'm expecting, but I have little doubt that the unified hardware - one platform for handheld/console will be true (though you probably have to buy both halves separately, because this is Nintendo.)
Also, if all that is true, releasing NX this year doesn't make sense, but I can't see them waiting either. All signs seem to point to it even though it's too soon. If it's two parts it might make sense the handheld part releases this year (replace the aging 3DS) and release the home part the following year. Or the other way around, replace the ailing Wii U this year, and keep 3DS momentum until the following year. Early games could possibly be dual release for both as many suspect Zelda U will be.
In any case it's a weird generational jump with timing because ultimately if all that is true, they're merging what was two platforms and two customer bases into one for the first time since 1989.
All that said, I love the Wii U & 3DS and will be exceedingly sad to see them go. Wii U only got into its groove recently, but 3DS may have replaced SNES as my favorite console of all time. It wasn't a great era to be a Nintendo investor, but it's been a great time to be a Nintendo gamer! And also, we have all those fond Iwata memories from the E3's and directs of the 3DS/WiiU era. :(
