The thing is the way you interact with a touchscreen vs. keyboard and mouse is pretty fundamentally different. Some think keyboard and mouse is going to die out - probably one day, but not yet. Others think you can design an interface that suits both modes, but it'll always have compromises.

So if you've got a device that you use in both modes (like a tablet with bluetooth keyboard and mouse, or a convertible laptop), what are your options? Dual-booting isn't ideal. Probably better to have a single OS with two distinct interfaces.