Homebrew developers have already added Wii U Pro Controller support (and Dual Shock 3, Dual Shock 4, GameCube - yes, the Wii U can play GameCube games) for Wii games in Wii mode. Nintendo could just as easily do the same (and basically that's all that they have done in this situation), and not make people re-buy games. Of course there's no money in that.