> Those are all RPC mechanisms, not an ABI

I'm not sure about the others, but COM is an ABI. There's a bunch of stuff surrounding it that is RPC-like but the core specification is just binary layouts and calling conventions. It's arguably more cross-language than the C ABI since it lets you generate type-safe bindings for any language, unlike in the latter where you need to parse header files.

> You could have an RPC protocol be your only interface to the OS, and that would be a valid design, but would have a performance cost

Maybe it would, but I doubt anybody would notice. The whole "everything is a file" concept on unix is basically just this (also X11/Wayland).