GameCube's issue was that they launched too late after the PS2, giving the market leader a full 18 month headstart was not smart. They needed to back off the N64 sooner, sure give it some games like Banjo-Tooie and Paper Mario to close things out, but Perfect Dark and Zelda: Majora's Mask should've been moved to the GameCube and they should have focused on launching it for holiday 2000.
Letting Sony build such a big headstart with PS2 and then having to launch alongside XBox with no advantage was a mistake.
Also whoever thought the purple lunchbox design was the way to go needed to be booted into outer space.