Why would they need a new OS? When Microsoft introduced the concept in 2002, they simply added the features into XP.
I used one of the early tablet/laptop convertible computers. Underpowered, underwhelming. The hybrid tablet/laptop has a thirty year history of failure. Microsoft kept trying it and kept failiing. A device that you can hold comfortably in your hand will never be the same device you want to sit in front of for hours doing work. Either the device will be too big to hold or the screen will be too small to work at for long periods. Since you're going to have different hardware anyway, you might as well have separate OSes and interfaces that take direct advantage of the hardware differences.