Even their worst console, Gamecube sold 22 million in less than 5 years. We're talking about a 7 year long generation here, and their biggest hits haven't even come around. Also the market is a lot larger now.

Another thing people continually overlook is the fact that the WiiU's retail price is still a full $100 more expensive than the Gamecube's launch price was.  In fact, it's still $50 more than what Wii launched at, and that was the most expensive console launch for Ninty at the time.

There are two things that WiiU does have going for it:  The fact that it will always remain the cheapest 8th gen console (particularly if there's ever a Gamepad-less SKU), and the fact that it will have the most unique library of games of the three.  These two things could very well help WiiU's "second system" status as its library continues to grow and the price eventually hits $199.

Personally, I think WiiU will certainly top Gamecube's sales and possibly outsell N64 when all is said and done.