I think this is a very debatable claim. MSVC is an insignificant compiler in a lot of industries. In something like scientific computing, everything is written with gcc, clang and Intel on mind. Sure, you'll find some companies still using MSVC, but it's not industry standard any more and it's very unclear how an insignificant, outdated compiler can stop the progress of world's most fundamental programming language.
C's strength is being the lingua franca of the bottom of the stack. I can believe that there are some applications that know a priori that they will never need to be portable to MSVC. But for any foundational open-source library, closing the door to MSVC is a high price to pay. Someone, sooner or later, will want to use the code on Windows. If you are zlib, ffmpeg, freetype, Lua, LuaJIT, sqlite, libpng, c-ares, OpenSSL, PostgreSQL, glib, gtk, or anything that aspires to be as widely used as these, you stay C89-compatible to support MSVC.
