In C's defence, you only have forward-compatability of string handling to the extent that your string type will not change in the future. Byte arrays are great for this. All string encodings in the foreseeable future can be encoded in bytes. You do not need to recompile coreutils once someone invents utf8-plus.

If you wrote your standard lib in whatever was better-than-C at the time, you might have settled for sized strings (who needs strings longer than 65535 bytes?) instead of \0, and you might have had utf32 or some kind of wide char representation.