C hardly requires it's runtime. Objective-C without the runtime is just C, if you write the runtime, GC, and other low-level system components in the C subset of Objective-C you are just writing the kernel in C.
This is in the original context of how all systems will be written in "GC enabled system programming languages". If the language is not GC enabled due to missing a runtime or w/e then the technicality isn't relevant.
The problem, is that at the end of the day, some code somewhere is going to have to deal with resource allocation. Generally speaking with all the other fluff aside, an operating system, fundamentally manages and multiplexes resources. It's naive to think that resource management would be best done in a language with automatic GC. Somebody
I don't doubt that C is not the systems programming language of the future. But it's not going to be done in a system that's based around automatic GC either.
You use the word proven as if it means something. The work you list is no more proven in terms of building production systems than any other research work.
Additionally, I find it interesting to note, that if you had in fact been very familiar with all the work you mention. You should actually have noted that many of these systems go through significant effort to sidestep the GC.
The same applies if you would be using the C subset of a C++ compiler.