As long as extensions are careful to stay "syntactically isolated", the complexity of parsing C need not explode. E.g., at least on Linux where glibc headers are very gcc-tilted, the only extension tcc really needs is that __attribute__(()) stuff (maybe 1 or 2 other "just ignore this stuff" things).
While there can always be edge cases that are broken, I use tcc daily on Linux without issue. So, I would encourage people to not view C as an untamable complexity monster or be captive to prevailing implementations. { C++ is, of course, a more unfortunate story in this and so many other regards :-) }
Rust will have Rust/WinRT on Windows, Nim can also go down that path for example.
Anyway, the world of C extensions has mostly seemed syntactically judicious/surgical. That's more just cultural, not guaranteed by anything, but it makes me suggest (without knowing) that MS extensions would probably not be hard to support without complexity explosion. Apple, on the other hand, doing their own chips now and Swift and just generally being even more of a closed system..They seem most likely to make that hard someday if they ever think they can make more money by not supporting POSIX-like programs. :-( { EDIT - there will likely always be some demand for/a way to create C callable wrappers/headers for system services, though. So, I don't think that it's crazy to hold out hope that this will remain possible/not too bad for the foreseeable future. }
In what concerns macOS, UNIX certification is anyway only for CLI applications and daemons like software, stuff like graphics isn't part of UNIX trademark.