Automatic memory accounting — construct/copy/destruct. You can't abstract these in C. You always have to call i_copied_the_string(&string) after copying the string and you always have to call the_string_is_out_of_scope_now(&string) just before it goes out of scope

This seems orthogonal to std::string. People who pick C do not want automatic memory management, but might want better strings.

Automatic memory management is literally what makes them better

For many string operations such as appending, inserting, overwriting etc. the memory management can be made automatic as well in C, and I think this is the main advantage. Just automatic free at scope end does not work (without extensions).

You can make strings (or bignums or matrices) more convenient than the C default but you can never make them as convenient as ints, while in C++ you can.

Yes, but I do not think this is a good thing. A programming language has to fulfill many requirements, and convenience for the programmer is not the most important.

Empirically it is. All the most used languages are the most convenient ones.