This.

That's the worst case scenario and still very little would happen.

This. They survived the GameCube, they can survive anything. I think that they only would have problems if a home console bombed hard and their handheld bombed too (wich definitively isn't the case with 3DS).

Man, do you really think that a console that sold 160 million units with a great tie ratio made less money than one that sold 25 million?

We were talking about Nintendo's "Core" audience so I didn't mention them. Plus, they are not really long-term franchises. As you said, Wii Fit was huge for the Wii but Wii Fit U probably won't be nearly as big.

I believe Nintendo is definitely working on the Nintendogs, Wii Sorts, Wii Fit, of the Wii U. It will likely use NFC or a new peripheral. If not, it will use the Gamepad in a way that nobody saw coming. Either way, I'm sure Nintendo is working on something.

First of all, the Wii U isn't really struggling right now. Remember, we have to wait until it has two Christmas's!

But if it does end up being the "Modern GameCube" then I still think it won't sell nearly as bad as the original GameCube. 40 Million minimum would be my prediction, and that's if times get really rough. Wii U should be able to pick up ground as long as Nintendo actively releases games for it and keeps it at an aggressive price point.
Overall I think they'll make a profit from the Wii U, it won't be the bane of their existence.

Nintendo's core audience can't lift their fortunes about a GC level.   If they could, then GC would have been more successful as it was more 'core' than Wii.

They need their expanded audience, but I think the dual analog gamepad is a huge deterient to that very audience which is why NintendoLand is no Wii Sports.

Hypothetically, if the Wii U ends as something of a flop for Nintendo, relatively speaking, what direction will they go next? Will they abandon motion controls, abandon the gamepad concept, or perhaps revisit a Wii situation where motion controls are the primary control type? Or will they scrap all of that and trot out a new gimmick/innovation?

To the Moon!

On a serious note, Nintendo will be fine. The Wii made Nintendo millions so they can survive a "failure" this gen even more than one gen and considering how well the 3DS is selling they aren't going anywhere. Like Salnax said, worst case scenario, the Wii U sells like the GC, but they will still make profit out of it (maybe not in the first year).  

Nintendo can take a 20 billion yen loss ($257 million) every year until 2052, they aren't going anywhere anytime soon. 

Right, Saturn just never got the necessary software support to keep it afloat.  Dearth of software is always what leads to the early death of a game platform.  They can say it was the specs being too difficult to program for, or the marketing being off, or the price, which could all be true, but at the end of the day it was that N64 and PSOne had lots of high-quality games that people wanted, and Saturn just didn't.

GameCube was profitable for Nintendo.  They made some money on the hardware itself, but combined with the software and accessories sales it was a very profitable business for them.  GBA and Pokemon contributed heavily to profits too, obviously.

The rumbling you hear is the Wii U getting ready to takeoff. Launch pad, Wii U Direct tomorrow, get ready.