> that there is only one C

IMHO one of C's strong points is the ubiquity of non-standard compiler extensions, C with Clang extensions is much more "powerful" than standard C, personally I see C as a collection of closely related languages with a common standardized core, and you can basically pick and choose how portabel (between compilers) you want to be versus using compiler-specific extensions for low level optimization work (and in a twisted way, Zig is also a very remote member of that language family because it integrates so well with C).

> "one notch up from assembly"

C is actually a high-level language, it's only low-level when compared to other high-level languages, but much closer to those than to assembly. Arguably Zig is in some places even lower-level than C because it's stricter where C has more "wiggle room" (for instance when it comes to implicit type conversions where Zig is extremely strict).

> being available on any platform, existing or future, any architecture, suitable for any bare metal use case

Zig has a pretty good shot at that requirement, it has a good bootstrapping story, and for the worst case it has (or will have) a C and LLVM (bitcode) backend.

For me personnally, Zig will most likely never completely replace C, but instead will augment it. I expect that most of my projects will be a mix of C, C++, ObjC and Zig (and Zig makes it easy to integrate all those languages into a single project).