Such a bounds-checking language extension needs to be able to annotate existing libraries without changing their API, otherwise it's not all that useful.
What I've noticed about C and it's problems with safety is the discussion always assumes that C will be replaced real soon now. So the problem is really about existing code bases.
After 40 years of that I think that was a bad assumption.
I also think that with annotations you can fix code mechanically.
Yes, easy to do and a good idea in your own code but maybe not an option in a library that needs to remain API and ABI compatible with the previous version of the library that used separate ptr and size arguments (for whatever reasons - for instance Apple might want to harden system libraries without breaking existing applications).
PS (too late for edit so I'm replying to myself): with the above Clang extension you can also define your own phat_ptr_t struct and still associate the length with the pointer. Not sure if that's also possible with the Microsoft extension, for instance this is copied from the proposal text:
typedef struct {
int *__counted_by(count) buf;
size_t count;
} sized_buf_t;
