Hypothetically, C could use the first `sizeof (size_t)` bytes to store the length. Endianness shouldn't be much of an issue; just use the endianness of the current machine. You don't typically write the NUL terminator when writing a string to a file; similarly, you wouldn't typically write the length field when writing a counted string to a file.

I agree that NUL-terminated strings were the right decision at the time (and aren't that bad now), but there are sane ways to do counted strings.