Wii U production will have ceased by then. It'd be more feasible to released it on the original Wii, lol, at least with that there is more than one factory in the world that can make its DRAM (and it doesn't happen to be owned by Sony). 

If something completely awful happens there is a 50x better chance the Mario game Miyamoto is speaking about would end up on the XBox Scorpio and PS4/Neo than Wii U, if it needs to go to another system, that will mean the NX has failed and as such Nintendo's time as a hardware marker would likely be at an end. 

But there's still 13million owners (even if NX is a success I don't see it beating 13 mil).

Not sure I'm making the connection here...

They will cease being a console manufacturer and sell their games on Playstation/Xbox/mobile.

By 2017/18 when this game releases? Doubtful, many people will have sold their system or shoved it into their closest. 

This is likely why NX is unified platform, in *case* the console flops like the Wii U did, Nintendo can just keep producing the games for the portable NX. 

I think Nintendo has actually thought through this entire exact scenario, in part because both the GameCube and Wii U did not sell well they have to consider what happens if the console NX follows the same path (or worse). 

Beyond that I don't even think this game would run on a Wii U ... I'm going to guess the NX is something very different from the GameCube, what you guys are saying would be akin to telling a GameCube owner in 2003 that Wii Sports is going to be Nintendo's next huge hit game and the GameCube owner replying "yeah well, maybe they can release it on the GameCube" not understanding what's about to happen. 

Not if they make it like Xenoblade Chronicles or LoZ Breath Of The Wild. Xeno Cron was on the Wii but it used the classic controller, and we already know LoZ can be played on both, however they have to remake the controls would be far simpler in a Mario game than an open world collect-a-thon LoZ is going to play out.

So making their games accessible to more people to purchase = going third party. It feels like there's something missing from this formula....

I get the sense this Mario is going to be something wildly different and totally tailored from the ground up for whatever new controller the NX has. 

Miyamoto mentioned this even like over a year ago, that the next Mario would be for the next system and it would be different/system-era defining. 

EDIT: Here's the quote from 2014 (!):

Since we first created Mario, people have compared him to Mickey Mouse. I’ve always said Mickey Mouse evolved with each evolution in animation. You saw Mickey Mouse each step of the way. From early on, I wanted Mario to be that character in the digital world, so that with each digital evolution, he was there to usher in the next era. I think that maybe when we release the next hardware system, you can look forward to seeing Mario take on a new role or in a new game.

This isn't just "another Mario game" this sounds like it's meant to be something that is era/system/generational defining (for better or worse). 

Isn't that kind of forcing meaning to a phrase though? Different/system defining could be interperated many ways.

If their hardware fails to sell, they can release their games on other company's hardware. Like Sega did after the Dreamcast failed.