I think the implication is that Apple can push an iOS upgrade with security holes, and without an open-source system, we can't know if it has security issues or

I think the implication is that Apple can push an iOS upgrade with security holes, and without an open-source system, we can't know if it has security issues or not. (At least that's the impression I've gleaned from the other comments) I don't own an iPhone, so I don't know how the update process works.