That's an interesting question. Windows 10 Pro is running two systems all the time: Win32 and Windows Runtime. UWP apps run in sandboxes under Windows Runtime,

That's an interesting question. Windows 10 Pro is running two systems all the time: Win32 and Windows Runtime. UWP apps run in sandboxes under Windows Runtime, not in Win32. Presumably there are some benefits from this, but it depends how many parts of Windows itself use Win32. For example, the new Settings app runs in Windows Runtime but it still hands some things off to the old Control Panel, which is a Win32 program. I'd assume the eventual goal is to move everything to Windows Runtime, leaving Win32 to wither and eventually die, at least in the consumer market. I suspect enterprises are too slow to change for that to happen in business market. Bear in mind that UWP apps will run very happily on the ARM-based Windows laptops that will arrive late this year, whereas Win32 programs will use a Rosetta-type emulation.