It's nice to have options, but really, some people need to learn to move on. If Wii 2 U has GameCube controller support.....I don't even know what to think anymore....
Even if the next system supports GC controllers (which it very well may, considering that it uses USB) I fail to see any issues with it. It's just an option after all. If you don't want to use it you won't have to. I'd personally love to see more retro adapters so I can play VC games using NES/SNES/N64/etc. controllers.
Yes, it's nice to have, but many people seem to have a prejudice idea that the GC controller is and always will be the best controller for any Smash, when they haven't even tried using Wii U's controllers yet for Smash IV. I'm not against options, but it's just strange how people just don't even wanna consider a different controller.