Assuming your premise is true, that the Wii U never takes off...

1. That means Nintendo will continue to lose money, after 2011's first Annual Loss ever
2. They technically can manage & navigate any major storm, with literally $10 Billion dollars in cash & short-term investments in the bank.  This means if Nintendo really wants to - they can weather a dissapointment with the Wii U, go dark for 2-3 years, and emerge with another console - if they really wanted to do this.  It would take a stubborn iron-resolve to remain a Hardware company.
3. But really, being a publicly traded institution, there would be tremendous investment pressure, and there are avenues right now Nintendo could take, to generate hundreds of millions of dollars in revenue.  Namely: bringing their beloved franchises to the masses, on iOS, Android, & competing platforms.  Mario, Pokemon, etc., are particularly well suited for Smart-Phone & Tablet gaming.  The day you can buy a Mario side-scroller for your iPhone is the day Nintendo triples or quadruples Angry Birds figures.  
4. At today's valuation Nintendo already is, but would increasingly become a prime Takeover Target, by say, a Disney or an Apple, who could acquire Nintendo for what amounts to chump-change to them.  Disney especially, after the LucasFilms, Marvel, & Pixar acquisitions - are chomping at the bit to get companies like Nintendo under their fold.  Their growth strategy is by acquisition and make no mistake, Nintendo is high up on that list.  With Apple's impending bid on the "war for your living-room", a company like Nintendo would be a perfect fit to come pre-packaged in every Apple iTV sold.
5. But really if Nintendo continued as an independent Hardware co., I think their next hand-held after the 3DS is a phone, and their next console is a GamePad that projects to the television.

So if the Wii U fails, Nintendo becomes hardware agnostic, or gets acquired.  If they do become hardware-agnostic they may still choose to produce their own hardware.

Sure there are.  At the very minimum there are 20 million, as demonstrated that the GameCube exceeded that number, and that was even before the popularity explosion of the cross-over games like New Super Mario Bros, Mario Kart and Animal Crossing series.  Those series have massive appeal to casuals that aren't non-gamers but aren't necessarily active gamers.  They're an inbetween.

If 3DS only primarily appeals to hardcore Nintendo fans, then there are at least 27 million of them already and the system is only two years old.

They made some money off the GameCube but probably not a ton. The GameCube was actually sold at a slight loss when it launched (about $20 loss IIRC) and they had to lose money again when they cut the price to $99, but had no choice because inventory was piling up in their warehouses that was sitting there unsold. 

Not only that, but sales of almost all their key franchises dropped a lot during the GameCube days ...

Super Mario 64 - 11.62 million WW  --- Super Mario Sunshine - 5.9 million WW

Zelda: OoT - 8 million WW   --- Zelda: Wind Waker - 4.6 million WW

Mario Kart 64 - 9 million WW --- Mario Kart: Double Dash - 7 million WW 

GoldenEye - 8 million WW --- Metroid Prime - 2 million WW

I kinda used GoldenEye and Metroid Prime as comparables, because I think Nintendo was hoping that Metroid Prime would be the GoldenEye for the GameCube. 

Super Smash Bros. sold more on the GCN, but it did come out fairly late in the N64 life cycle. 

I don't think any of the above are likely scenarios within the next ten years except for maybe #5.  But that doesn't mean they changed their business model it could just mean those two products are the natural evolution of where those types of devices are going.

Hopefully this doesn't happen:

You're forgetting the price cut to $149 in May 2002, seven months after launch.  Which I suppose indicates that they were able to significantly reduce the manufacturing cost of the system in a very short amount of time.  I assume that was when they finally out-sourced manufacturing to places like China and Russia and didn't have to pay exorbitant wages for manufacturing in Japan. 

That would be a direct violation of PSP Go's copyright laws.

My prediction was and will continue to be that Nintendo releases a successor to 3DS in 2015 and that it's Game Boy brand and not DS.

Probably similar in looks to an Experia Z but with all the buttons of the 3DS, but only one screen, and it's multi-touch and has it's own proprietary Nintendo OS, not iOS or Android. Or at the very least, a modified version of iOS or Android that is heavily-restricted and focuses on eShop, Miiverse, and games, and cell phone functionality is there but takes a back seat.

They can subsidize just like HTC, Motorola, and Sony do by selling it optionally through major phone carriers for a monthly phone bill instead of $599 retail to recoup the cost of manufacturing.

The phone/handheld has a slot for 3DS games, but the game cards stick out slightly as the system is so compact. It's larger than most phones to accomodate all the buttons and the two mini circle pads, but it can be used just as a smartphone.

The catch to all this is that Nintendo games only run on this phone, and not on any other iOS or Android, so Nintendo games don't lose their long-standing history of being controlled with buttons and not swiping. They can call it the Game Boy X or something. Would sell like hotcakes. Merging a Nintendo handheld and a top-of-the-line smartphone? Ca-ching!

I could see Disney attempting a buy-over if the stock price goes low enough, but I would imagine that Nintendo would react very unfavorably to that. 

A Nintendo platform with Mario, Pokemon, Zelda, Marvel Comics (The Avengers, Hulk, X-Men,  etc.), Star Wars, Indiana Jones, Disney classic franchises, Pirates of the Caribbean, Tron, and Pixar properties all potentially exclusive would be something else though. 

An hostile takeover (and this one would be hostile) wouldn't be smart. The key members of the Co could leave, specially Miyamoto, leaving a shell with half the talant that it had. And if that happens, what's the point of having all those IPs if you ruin them?

Look at what happened with Rare's sell, and that was amicable. Are their IPs still atractive now? Not even half of what they were.