The gamecube wasn't underpowered.
Yes you are right. The gamecube was only slightly under power compared to the Xbox. But it was underpowered with features. People really cared at the time that it didn't play DVDs and that the game disks had lower capacity. It had the worst online play support and Nintendo was openly hostile to the importance of online play. The memory cards had like 1/8 the capacity for game saves, with no hard drive support. And it had the worst controllers, at least in the eyes of the majority, with weird shaped buttons, fewer overall buttons, and a stubby right stick.
Lifetime Sales Prediction - 6/29/2013
Wii U - 38 million
XBOX One - 88 million
Playstation 4 - 145 million