"I'm sure that the decline wouldn't have been as bad had Nintendo released more core IP, but my argument is that it wouldn't have made THAT much of a difference. The Gamecube was a console aimed at the core and it's sales reflect, within reason, how well hard core gamers receive that company. Had the Wii been released with a regular controller, I'm positive we'd all be talking about Nintendo going out of business or going stricktly handheld. That's how bleak it was back then. Nintendo KNEW this, and that's why they took such a massive gamble to begin with. IMO, Nintendo is going to be third this generation again because they never really advanced past that position with hard core gamers and for the most part, that's going to be who buys into next gen....core gamers."
First, if we have to use labels, I distinguish "core" gamers - from "majority" gamers - from "casual" gamers: Core being the hardcore Dark Souls players that live for deep experiences; Majority gamers being the ones who are definitely gamers, but have no allegiance/hate and like a lot of variety (Mario, Halo, Gran Turismo); Casuals being the whim buyers and soccer moms. Majority gamers are where the biggest sales come from as they are many in numbers and still buy software pretty regularly.
Many majority gamers moved away from Nintendo since the N64 era because of the many mistakes Nintendo made. While many enjoyed Mario 64, Zelda OoT and Goldeneye, the problem was there were so many games they couldn't get due to the cartridge format being such an obstacle for 3rd parties. There was no Metal Gear, Final Fantasy, Street Fighter, and N64 Castlevania sucked... all IPs that did great on the NES and SNES that were now on the Playstation; a console that Sony handled and marketed very, very well and successfully courted these "majority gamers". With Gamecube, first, it launched a year after the PS2 already had a commanding lead with majority gamers. Second, Nintendo tried to fight Sony on their terms... with a purple lunchbox. I mean, I loved the GC, but I totally understand why the market rejected it.
In the days of the NES and SNES, generally all gamers loved Nintendo, despite their family-friendly image. People weren't self concious at all about sharing Mario on the same console as Contra, Metal Gear, Final Fantasy, Ninja Gaiden, Castlevania, etc. Street Fighter 2 - the biggest 3rd party game for its gen - sold much better on the SNES than the Genesis despite Sega's attempts to shine a kiddy light on Nintendo. So Nintendo is no stranger to housing these types of gamers, and there is no reason that they can't appeal to them again if they play their cards right with what the market wants (including 3rd party games). That doesn't mean that these gamers will ignore Durango or Orbis at all, but these kinds of consumers are often the ones who buy more than one console anyway. And I'm not talking about Nintendo-haters; Nintendo can't court these people and obviously doesn't need to for success. I'm talking about the majority gamers who made the NES (90%+ market leader), the PS1 and the PS2 giant successes.
"The casuals are playing farmville and Temple Run."
This is a fallacy I see very commonly on forums today. The statement itself isn't wrong, it's the idea that casuals, hipsters, whatever you want to call them are gone forever, swallowed up by a giant ios/android black hole never to return. Many casuals bought the Wii because it was the "in" thing and "cool" to have, yes, but it also, most importantly, offered a fun experience for them. People didn't all of a sudden stop seeking fun experiences. If Nintendo delivers fun, unique experiences at an affordable price, there's no reason to believe that many of these consumers won't return if WiiU becomes the "in" thing in the market once again, which is what Nintendo is banking on.