At best all they could do is change literally every single private function call everywhere to inspect the return address and see if that return address is in a

At best all they could do is change literally every single private function call everywhere to inspect the return address and see if that return address is in a system framework or is in the app image. But this would be huge overhead, a real pain in the ass, and also not even reliable, because all you have to do is pick a public function, figure out the address of the `ret` instruction, push your $pc onto the stack, and then call the private function passing the address of that `ret` instruction as the return address. The private function will see that this address is in a system framework, and so will work, and then it will return, passing control to that `ret` instruction which immediately returns back to your real caller function. So no, there's no way for Apple to technologically make it impossible to call private functions. The only actual solution there would be to completely rewrite the OS such that literally every call into a framework that an app makes actually goes over IPC (so that way apps can't even attempt to invoke private functions since they won't be linked into the app), but that would probably be crazy slow which is why nobody does that.