NX was hinted at before even then back in 2014's investor's meeting in February.

Some difference will probably be there, for instance Wii version had 16:9 support and motion controls and mirrored world.

If you implying Nintendo will again delayed Zelda U and released in 2017, that will not happen because first meant to be released in 2015. but was delayed probably for whole year, posible one of reasons for delay was cross platform launch for Wii U and NX.

Huge difference is that Wii U had so many mistakes done by Nintendo (that are well known), and only people who buyed Wii U are mostly Nintendo fans, NX probably will probably not have majority of those mistakes and Nintendo will probably aim to make far more appealing, similar to Wii.

Whether or not a console has ever been replaced while it was at $300 before is kind of irrelevant.

For one, this is the first generation where all competitors launched with SKUs that were above $300. Hell, the only SKU that was released at that price, the 8 gig Wii U, was discontinued. So OF COURSE consoles being replaced while still at that price doesn't have much precedence.

Also, the Wii U is overdue for a price cut. The system has been at $300 since September of 2013. It getting a price cut this year is not only likely, but should be expected, and could very well be substantial.

Beyond that, a major reason for the Wii U's high price is its gamepad. A Wii U successor would be likely to replace that with a cheaper gimmick (if any) and more graphical power.

Finally, being $300 at the beginning of a year where a successor console will be released isn't unprecedented. PS3 had a $300 standalone SKU released in October of 2012. It was most definitely still $300 the following January of 2013.

Really, the best argument against a Wii U replacement is the simple fact that Nintendo hasn't ever cut a home console's life short before. Sure, they've done it to the semi-portable Virtual Boy, and the GBA was replaced early in response to PSP (though unlike most Nintendo consoles, Nintendo supported it well after its successor was released), but never to a home console.

Nintendo has a history of announcing games well before titles release, if they were planning to support it this year then they would have a fair few games slated for release this year, but that hasn't happened, not at every major event last year.
The OP says that Nintendo haven't had a robust line-up for Wii U until 2015, but if you look at 2013, 14 and 15 they were all about equal for volume of releases, yet in comparison to those years 2016 is barren, except for Zelda, which still has virtually no details and Star Fox which despite looking pretty good at the last showing had to be pushed back to nearer the middle of Spring and is also likely going to get pushed further back.

There's very little in the way of indies or even any of the few multiplats Wii U was still getting.
If you look at the 3DS line-up and Wii U together bother have actually dropped significantly in volume, this actually supports Nintendo moving on from both, since they always drop support for older platforms the year they plan to release their next system(s).

Nintendo aren't just replacing Wii U, they're replacing both it and 3DS, with one unified system and because they have issues in making games for 2 platforms they're going to release a new platform that will unify their game development.

Sony and Microsoft showed Nintendo that you don't need to drag out your launch marketing for a new piece of hardware, you can announce early that year and release the same one.
Going unified allows for you to consolidate your development of games and produce way more software.

The most telling thing is the line-up for 2016 for both Nintendo platforms, also the fact that even though Miyamoto was fine to announce the next Pikmin, but he wouldn't even confirm it as a Wii U game. Miyamoto's own words were that Pikmin 4 was "close to completition", yet he had no gameplay to show at E3 or another 2015 direct, if it was a Wii U game he would have shown it and said so, but he didn't.

Everything points to Nintendo release a new console and a new handheld this year, both are probably part of a new family of systems, since Iwata confirmed this to be the direction that he and Nintendo in general wanted to go.

I wasn't implying that Zelda U was going to be delayed again, I was simply saying that, as it is with most delays, the game is simply being improved and such.

Problem is they have the reputation of the WiiU shadowing over it.  They need to break that and making the same release scheduling mistake (yes it would be a mistake) will do little to prove that things are different.  Also they will never recapture the Wii audience.  That hit to the casual market was a one hit wonder as they have all moved to mobile, so targeting that audience would hold it back as well.

Yes, but hinting that a follow up is already in development is like saying that the sky is blue. The next gen is usually on the drawing boards pretty soon after the current gen is released.

Definitely, game will probably be more polished and have more content and side quests in that huge world when will be released at end of this year, than that was released one year earlier.

It wouldnt be same relase scheduling beacuse Wii U was relase only one year before PS4/X1, while NX will be relase at least 2 years before PS5/X2 and probably around 3 years, basicly in middle of generation.

They will not aim just casual market, they will aim core and casuals in same time, with Wii they aimed casuals, with Wii U they aimed core gamers, but Wii U had to many big problems to be successful and popular console.