A separate mobile OS with a different UI for apps would mean having to establish an entirely new app ecosystem, which at present is effectively impossible (if it isn’t just another Android clone). It would probably have been possible in the early days of Windows mobile, but they botched that for various reasons. If Microsoft wants to have an OS in the mobile/touch realm (I’m not saying they should), sharing apps with regular Windows is their only chance.
IMO, they need to establish a more structured and controlled UI layer/framework that would allow (or rather, enforce) auto-switching between touch and non-touch layouts, with a uniform look&feel across applications, while being as performant/reactive as win32/GDI applications, and then stick to that for the long run, so that it becomes the one Windows UI toolkit (like GDI used to be), similar to what Apple has on their OSs.