MK doesn't need touched, but the smart move for them and the splatoon community would be making splatoon cross play and the same game across both platforms. This benefits them, as they want the game to have a large competitive community and an esports scene.
Near 3 mil install base on Wii U splatoon already, pretty stupid to ignore that if you want to push it as a serious competitive community.
Mario Kart update needs to be much bigger then the splatoon one, and probably shouldn't bother with Wii U at all tbh.