Also consider that this extension is designed in a way that existing code can be annotated without requiring drastic changes, and that it has been designed to remain ABI compatible.
A 100% watertight solution most likely requires new language features (or even a completely new language like a "Rust--") that would violate both of those requirements.
It is only C if it lands on the standard, otherwise it is yet another compiler specific language extension, that portable code cannot rely on.
For that Clang extension above it looks like it's possible to annotate source code without breaking compilers that don't support the extension by defining a handful of dummy macros.
IMHO the actual strength of C is that compilers can (and do) explore beyond the standard on their own.
It is definitely a problem when said extensions don't have a counterpart in other compilers, specially a pervasive feature like bounds checking.