Was Wii Fit a big system seller? Not doubting that comments reality, just asking a serious question. I know Wii Sports was. I didn't know Wii Fit was.
Personally I don't think either would have the impact of Wii Sports on Wii simply because it was something new and had a certain charm that is very hard for developers to recreate. Also, WiiU is not popular. And you have the Kinect now.