Why Microsoft had to combine two operating systems into one?

I'm referring to the situation of a windows (suitable for tablets) and a windows (suitable for desktops) into a single OS, windows 8.

There is no doubt that windows 8's interface is a mess. It's not that you can get use to it. It's that metro apps are pointless when using windows only in desktop mode. Sure, it opens opportunities for new hybrid devices, but it came at the cost of downgrading the quality of windows. I am very sure that Microsoft was well aware of this problem (then again there not that stupid) and did not mix the 2 OS just to make room for hybrid devices that can do a lot but nothing really properly.

I think the real intention of Microsoft was avoiding the situation of Windows Phone. See, Windows Phone is a great OS, but it has a small user base, because it lacks apps. But Developers don't develop any apps because it has a small user base. It's like a deadlock. Now, Microsoft wanted to enter the tablet market. If they made an OS that was made just for tablets, they'd have the same situation as Windows Phone. So instead, they thought, we will combine the two OS so we can use the (potentially) great user base of Windows 8 to intrigue developers to make apps for App Market (or whatever it's called). What is your take? Why do you think Microsoft combined 2 windows into 1?