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

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.