Why wouldn't it run in the appropriate mode based on usage? Full OSX when docked, iOS when undocked? MS demo'd something back in the day with Windows Phone wh

Why wouldn't it run in the appropriate mode based on usage? Full OSX when docked, iOS when undocked? MS demo'd something back in the day with Windows Phone which I thought would be awesome. I thought one laptop MFG had actually productized it but I can't find it.