> because of the 16-bit character type

That type is not for characters anymore; all these languages treat these values as UTF-16 units. Unlike C++, their standard libraries provide functions to convert strings between UTF-8 and UTF-16, apply Unicode normalization, decode strings into a sequence of code points, etc.

> That's what iostreams are

iostreams tried to implement binary streams, text streams, and object formatting with the same API. Predictably failed all these tasks, the features are too different. A minimalistic C++ API for a stream of bytes might look something like that:

    struct iStream
    {
        virtual ~iStream() { }
        virtual void read( std::span<uint8_t> buffer ) = 0;
        virtual void write( std::basic_string_view<uint8_t> buffer ) = 0;
    };

> result in developers making software that doesn't work like users of the OS expect

Can you elaborate? When I write string content = File.ReadAllText( path ) in C#, the standard library does exactly the same thing as some C++ library could do: open file for read access, read it to the end, and because C# strings are UTF-16 convert the bytes from UTF-8 to UTF-16.

> Async interfaces are all the rage these days. Meanwhile browsers have moved from TCP to QUIC

I think both points are exotic stuff. Not sure I would want to see them in C++ standard library. Also going to be hard to design a useful platform-agnostic abstractions. OTOH, Unicode strings, file systems, and streams of bytes are literally everywhere in software.