There is a terrific benefit to learning on a platform where you can not only look up documentation within the system (man pages), but also "dig deep" into the code of any particular piece of the system you are interested in. So in learning C, you gain access to the body of knowledge contained in a Linux system, or contained in a BSD, and when you get curious about something like "These loops and conditionals are all fine and well, but how does a window actually get drawn to my screen?" you have everything at your fingertips to go and answer it.
Is that connection to the machine possible without learning C? You could use a toy operating system written in something else, but you miss out on the honesty that comes from teaching the same system that people are actually using (or at least able to use).
I think Go might not be an unreasonable alternative. It shares C's philosophical heritage, removes the need for manual memory management, and is capable of exposing a student to pointers, creating images, and HTTP communication all within the stdlib and the first chapter of the official book. The only downside, as I see it, is that one would need to additionally learn something like Rust to be able to apply the concepts to things like kernel drivers and close-to-the-metal programming.
