You're both right. It's the kernel's job to ultimately enforce this policy, and calling private APIs should only be a compatibility problem, not a security risk

You're both right. It's the kernel's job to ultimately enforce this policy, and calling private APIs should only be a compatibility problem, not a security risk. I believe the current sandbox design makes it more complex to promote system interfaces to require individual access control. It doesn't provide the fine-grained access, so more work is needed to separate code with the granularity that sandbox policies can control it. This does happen over time (e.g. UDID in iOS 8).