If you know this language, you can change any part of OS. I think that's a valuable asset. For example if you're using OpenBSD and know C, you can hack anything from kernel to smtp daemon. If you're OS is written with 10 languages, it's much harder to learn them all.
It never is. Even ancient unix had shell and (maybe more to your point) yacc. Yacc still generates C files, but that's more of an implementation detail. The C _interop_ is key.
Well, of course! I was talking about why yacc is a second language for (early and contemporary) unix. Its C interop for (early and contemporary) unix is what I was stressing.
