I think how high SMOs lifetime sales will depend largely on whether or not Nintendo does a new 2d Mario for the system or not. We might just get a mario maker for switch instead of any new 2d platformer. If that happens then I'd say the sky is the limit for SMO because there will be no proper 2d mario to overshadow 3D mario in sales like what usually happens. It could definitely go 20m or even higher in that case.