Let's say that Australia wants to ban consumer encryption. This would currently be difficult to enforce for PC software. But on mobile, this is easy. Just make Apple and Google enforce it! Make them ban such apps from their stores. Now you've achieved perfect enforcement on Apple hardware. Even on Android, where people could in theory side-load the banned apps, this would prevent those apps from achieving any scale or network effect.
That's what I think people are missing here. No matter how much you trust Apple, once the mechanisms for this kind of power are in place, you won't be able to control what happens next.