I seriously doubt these people who talk about third party games missing from Nintendo consoles would even get the Nintendo console if they released an equally powerful console to Sony and Microsoft. Look at the Gamecube and N64 era, third parties didn't support Nintendo then either and gamers didn't buy them much more than they're buying the consoles now.
Only reason they did buy the console more was because there wasn't a clear winner. And when I say clear winner, I mean a console everyone knows has won the generation a month out of the gate. Gaming media has jumped ahead by leaps and bounds these days and IGN is a household name for any gamers out there.
Chances are if you wanna play third party games like Assassins Creed or Metal Gear Solid then you probably aren't a big fan of Nintendo style games. Nintendo would be stupid to try and depend on third parties again. Even if Playstation and Xbox weren't a factor anymore and Nintendo was the only console on the market, gamers would simply go to the PC and Nintendo sales would go back up to Super Nintendo numbers.
Nintendo's strength is that they're innovative and they take risks. Those risks either pay off big time like the Wii or fall flat like the Wii U. Nintendo's Treehouse event at E3 last year is a perfect example, no other company would bother trying crazy stuff like that.