> C is the natural language for exposing Unix system calls
No, C is the language _designed_ to write UNIX. Unix is older than C, C was designed to write it and that's why all UNIX APIs follow C conventions. It's obvious that when you design something for a system it will have its best APIs in the language the system is written in.
C has also multiple weird and quirky APIs that suck, especially in the ISO C libc.
If I write an OS in Basic, surely the 'natural' language for exposing the system calls is Basic?
Yes Unix predates C. But at this point in time 50+ years down the road, where the majority on nix users don't use anything that ever contained that code, and the minority use a nix that has been thoroughly ship of Theseused, Unix is to all intents and purposes a C operating system.