I agree with comments about GCN controller complexity holding back sales (and maaaybe the lack of motion control although I'm not sure that would have bothered people that much). That said I think it would easily have been the top seller and would have driven hardware.
I think it would have crossed the 10 million line, but probably not by a lot - 10 to 12 million. I think it also would have pushed an extra ~5 million GameCubes.