I think that some points are missed here.
A compiler is a compiler that means a tool able to input a C grammar file and output executable code which executes 1:1 what is described in the source.
Based on this any compiler can produce code suitable for applications even for different target (see my sample OS with cross compiler using PellesC
http://forum.pellesc.de/index.php?topic=443.0).
Of course MS has introduced a lot of 'extensions' to the language, but they are almost all implemented in PellesC.
So first point: you don't need access to compiler code to import libraries, or modify or extend existing ones, to the compiling environment. That simply requires some advanced knowledge and OS familiarity. And of course a lot of time
(that unfortunately I really miss in the last times).
MS is not moving away from C simply because there is no other language so efficient, low light and flexible as C to program fast and superior system code.
The real point is that with other languages they can supply user programming interfaces that already contains almost whatever function you want. This way some are happy because even if they are not so bright can show the "hello world" window with a couple of clicks and get proud of theyr knowledge of IS (have you noted that most of the people that carry the last model of smartphone and talk, talk, talk, talk about computers generally barely knows what a keyboar is for...
).
On the other hand professional users get advantages from fast project deployment and ready tested code (even if with MS you cannot bet on this......).
For this reason MS expose only the interfaces for that languages, even if the core of the code is still in C (maybe not true for WEB tools because thy run on your browser, and a slow system is a good leverage to make you buy a new computer or a new OS....
).
Sometimes the API's are just wrappers for other languages, sometimes that functions can be reproduced using low level API's.
A the end if you write something 'reusable' don't care about the compiler, one or the other, unless a really broken one, will work....