As a professional C programmer, I really couldn't care less whether or not Microsoft decides to implement C11 or remain in the dark ages. That ship sailed years ago.
You can even use Clang on Windows if you need (MinGW, Cygwin or compiled with Visual Studio). I'm not sure if you can integrate Clang in VS for example, or if it is possible to access the OS C libraries from Clang.
But there is a ton of software being written right now that doesn't target Windows, with no plans to target Windows. The only way Windows users will ever be able to use that software is if Microsoft makes it easy enough to port.
The nail in the coffin will be when most of the games on Steam do not work in Windows. With most business software becoming web-based, the only holdouts left will be old businesses locked into their enterprise crapware.
https://github.com/rbultje/c99-to-c89
(see http://blogs.gnome.org/rbultje/2012/09/27/microsoft-visual-s...)