Eh... all the major OSs use C, anyways. For starters, (almost?) everyone is using a decent bit of assembly at the bottom. Then Linux is slowly adding Rust, and I was under the impression that NT spanned C, C++, and maybe some Rust now? And the moment we speak of the whole operating system and not just the kernel I think your claim collapses completely in the face of Darwin's Objective C and Windows doing... everything Microsoft feels like including.