XBox (the first one) really honestly should've sold 30 million or so if MS had not discontinued it prematurely too. It was outselling the GCN by a decent margin every month down the stretch.
Wii was a bit of a fluke IMO, Nintendo's management has shown they're not nearly as smart as that so some luck had to be involved there. That plus they didn't have to compete against Apple/Google's app marketplace, from the time those have taken off (around late 2009) Nintendo has pretty much lost all the casual market. They can't compete against Apple.
The iPhone/iPad has brought gaming to far more casuals than the Wii could ever dream of and IMO keeps them consistently playing more often too, not the whole "we only play the Wii once or twice every month when the family gets together" thing.
