First Microsoft will have to move people to Windows 10 (currently at less than 15% market share).

True, but Windows 10 has more than 200 million users, which makes it bigger than all versions of Mac OS X and Linux combined.

The US DoD has already started to move 4 million users to Windows 10, which is a good sign.

Most businesses will be planning to move by 2020, when Microsoft stops supporting Windows 7 (though I expect a few daring souls will be hoping for an XP-style extension).

So, Windows 10 is slightly ahead of track for a billion users at the moment.

Just how many gamers are we talking about here?

34% Steam users run Windows 10.

http://store.steampowered.com/hwsurvey

A more relevant statistic, thank you, but also a reminder that Steam is another obstacle to UWP.

Would developers prefer their games to be on Steam or on the Windows Store?

Their choice: they could do either, neither, or both.

Presumably Tim Sweeney wants to do UWP or he wouldn't be making such a fuss about it....

Maybe some developers will think it's a good thing if Steam has some competition?

How can I allocate executable memory to write a JIT compiler under UWP?

Most likely by having the right appx permissions to do so.

Project Centennial is supposed to allow for 100% compatibility with existing Win32 APIs.

This is from last BUILD.

https://channel9.msdn.com/Events/Build/2015/2-692

https://channel9.msdn.com/Shows/msdevshow/Episode-56-Project...

This upcoming BUILD might have more talks about it.