Surface Book isn't designed to be mass market, Surface Studio isn't designed to be mass market. Maybe not even Surface Pro when it first launched, who wanted a touch screen windows? Can it be mass market? Yes. It can take off and maybe people realize they wanted dual screen devices all along. It doesn't mean they start designing devices with "how many people already use this device". If you want to argue about the semantics of "mass market", I'm not interested in that discussion because I think you get what I mean.
Have you tried multi-tasking on a smaller tablet like an iPad Mini before? Have you ever used a dual monitor setup before? Two screens offer very different advantages compared to just one big screen.
