You say that like they are removing all C support. Unless that "legacy" code suddenly mutates to use C99 features that aren't in C++, it'll still compile in the future.
Forcing legacy code to stay woefully out of date to support a single vendor's stubbornness, perpetually growing its maintenance burden as the rest of the world moves on. Hooray for the Microsoft way!
C90 isn't going away, and it's not Microsoft's fault. Many C compilers still support 1st edition K&R style for a reason — old C code is going to stick around, and it's probably not going to change much, let alone in any way that needs C99 features.
Ad 2) If there is support for features there will be people using them.