It's not unusual for an operating system to require its programs to use its system calls. A Unix program that doesn't speak Unix protocols is just as useless as

It's not unusual for an operating system to require its programs to use its system calls. A Unix program that doesn't speak Unix protocols is just as useless as a Plan 9 program that doesn't speak Plan 9 protocols. It's a moot point though, since you'd have a difficult time unwittingly making a compiler generate a program that doesn't work on its own operating system. Pipes on Plan 9 use the same pipe, read, write, and close system calls as Unix does; they are an equivalent amount of work to use (or adopt, as you say).