But is the "proper UNIX way" a good thing?

Funny that you would be arguing for that (unless I misunderstood the intention), given your many other posts about how C is a horrible broken unsafe language that should not be used by anyone ever. I tend to agree with that, btw, even if not so much with the "memory safety" hysteria.

Should every program, now and in the future, be forced to depend on libc, just because it's "grandfathered in"?

IMO, Linux is superior because you are in fact free to ignore libc, and directly interface with the kernel. Which is of course also written in C, but that's still one less layer of crud. Syscalls returning error codes directly instead of putting them into a thread-local variable would be one example of that.

Should a hypothetical future OS written in Rust (or Ada, ALGOL-68, BLISS, ...) implement its own libc and force userspace appplications to go through it, just because that's "proper"?