Comment by nanolith

What you say is true, but you've inserted an inaccurate context with the quote.

There is no guarantee that _direct access to system calls_ will last forever...

Stability in the syscall API exists because Linux is a kernel that supports multiple distributions. Not because random applications could call it. The latter is an emergent feature, but not one that distribution maintainers will necessarily respect. POSIX only guarantees access to functions in libc that can perform these calls. As mentioned elsewhere in this thread, there are specific reasons why future direct access to system calls in user code could be restricted. Whether they will or not comes down to how distribution maintainers decide to deal with syscall related ROP gadgets.