On it's current trajectory, Wii U probably won't outsell GameCube.
It's entirely possible for it to outsell the Cube, but Nintendo would have to make a big switch in strategy: less GameCube-like software, more "casual" and old-school software, and way more software in general, also start wrangling in key third party games like Minecraft, Call of Duty, and so on...not just Japanese ones.
I don't know think it's likely Nintendo will take those steps.
I actually think Nintendo could outsell N64 if it really pushed hard but it would likely take a loss or make no money which is not Nintendo's style. I think it could probably sell around 30 million units and still make a small profit if it made all the right choices.







