I see. It was already in production. It's an open question whether the more complex syscall conventions would have held up over time as new features were added.

I see. It was already in production. It's an open question whether the more complex syscall conventions would have held up over time as new features were added. It makes a lot of sense in general to push unnecessary complexity out of the kernel and into userspace, since processes in userspace are allowed to fail. The complexity of the system call interface can be hidden behind a library.