What happens if you truncate your string? You lose information about buffer size. So now you need to store two sizes for such strings to be useful, string size and buffer size, which is 16 bytes for size_t on 64-bit systems. On top of that, strings are no longer arrays. So either the language would have to incorporate first class support for these strings, or you'll have people extracting the string pointer from the struct to perform indexing operations on themselves.
The approach obviously only works for read access and not for mutation. However those are the most often required operations. For owned strings and mutations different APIs are required. C++ string_view vs string, and Rusts str slices vs owned Strings work like this.
