Apple uses software to sell hardware, and usually stops supporting other platforms when it buys things. That's how its closed, proprietary world works.

Microsoft makes its money from software not hardware, and most of it comes via third parties not direct sales. It licenses most of its software across multiple brands of hardware, and it's been supporting non-Windows platforms (in the form of Mac OS and iOS) for a long time. It's already hosting Linux on Azure.

It's not going to close off markets for doctrinaire reasons, only if there's no money in them...

I really doubt Microsoft is going to make it Windows Phone only. Windows Phone is just not that valuable as a platform.