> But again, why, what do you gain?

Performance? Codegen simplicity? Why, again, must one use the syscall ABI for anything that is not a syscall?

> that ABI presumably has a representation for strings and allows passing them around

In this particular case, the API operates with binary buffers, not text strings. Sure, you can go the VMS way, or even IBM way, and turn files from binary blobs into arrays of fixed-length records (that's why C's fwrite/fread have both num and sz arguments: some OSes literally can't write data any other way).

> At which point we're back at needing a way to write strings in an in-memory format to OS-standard files in the filesystem.

Yes? Some text editors converted LFs to NULs to work on the text in memory, and then they'd convert NULs back into LFs on writing to the disk (IIRC). Both emacs and vi don't store text in memory the way it's layed out in the file; they translate it when writing to the disk.

Again, why do you want the OS to get involved into any of this? It's not the OS's job, period, stop trying to make the world an even worse place.