I still haven't heard a compelling argument for why Nintendo, or Sony for that matter, will suddenly fail. The lack of knowledge about what they'll offer going forward isn't a viable argument for their demise. Its a reason to be sceptical at best IMO.
Your essentially saying that if Nintendo could find success with gc, that they can compete with wii2.
Let's take a look at where the market was at the time:
-MS was a new comer to the market, nobody knew how committed they were and what the software library would look like.
-Sony had a year head start and pulled in major support from most everyone in the industry.
-Sega had just been stripped of their hardware going forward and relegated to a software only company.
Nintendo priced their machine at a discount ($200) in comparison to Sony and MS ($300) which made up for most of their lack of expected games library by weak 3rd party support.
3rd party support continued to dwindle as the years went on and the gen was shaped by ps2 domination.
Nintendo was smart about the design of gc though which enabled profit even with a discounted price to their competition and roughly equal graphics to the competition. Basically forming a Nintendo box to play nintendo games on.
This gen, wii was pulling in cautious support at launch which gathered steam as uncertainty in ps3 sales continued. However, sales did not follow for 3rd party devs. Now with competition coming in the form of Move and Kinect on machines that have shown sales of 3rd party software to be healthy, I expect 3rd party support to follow Sony and MS until sales show their hand.
What's different going forward is games development costs are going up. Games hardware costs are going up. This while the market for Nintendo software is limited.
It really boils down to how much support there is for a strictly Nintendo box.
Strip away the sales of Wii fit, and other motion games which no longer have an advantage nextgen.
How many will buy the wii2 on the strength of Nintendo software alone while there are other (better) motion alternatives on the shelf?
I say that market is shrinking.
Casuals
Nintendo has done a remarkable job in marketing Wii to the casual crowd. They have filled the void that the casual gamer was looking for in affordable gaming that also offered something new, and expanded into soccer moms and everyone in between. However, I don't see any loyalty from these consumers. A better product can easily pull them.
Hardcore
That market was in Nintendo's pocket back when they were the only game in town with nes. Shrunk a bit with Snes. And even further with n64. Further still with GC. And now completely lost with Wii.
Nintendo Fans
Been there since the beginning, but shrinking IMO due to age demographics (gamers getting older and outgrowing Nintendo offerings).
Granted, Nintendo is great with the kids, but the age of buying a Nintendo to have a "family console" is coming to a close with Kinect and Move offerings. So how much will it take to supplant the kid demographic with offerings by Sony and MS? Sony was already doing a great job of this with ps2. Software and hardware sales support this. Granted, GC was not a money loss for Nintendo in this competitive environment, but the market moves on.
Games get more expensive to make, systems get more expensive to make, and consumers expect more.
Nintendo is left holding the bag of hardcore N fans which may surprise me and be enough to keep them profitable next gen with Wii2. But how much longer will these fans be able to keep them afloat?
By 2020, they will be either moving on to greener pastures (younger kids & education), or teaming with someone to keep them competitive.