You're implying that exposing syscalls as the ABI instead of a higher level ABI is the only way to get backwards compatibility but that's not true. OS X can change its syscalls without breaking userland in exactly the way GP says: don't allow static linking of e.g. libSystem and its ABI is the ABI you're expected to use instead of directly calling syscalls.

Sure, Linux makes promises about its syscalls. But that's not the only way to get robust backwards compatibility and as GP says it's the oddity in that respect.