Could Apple not push an OS update that can compromise everything they are doing to make the iPhone unhackable? As long as user has to trust Apple there's always going to be the possibility that FBI/NSA/Whoever force Apple to update a target's iPhone to enable tracking/recording of whatever information.
It's not an attainable goal in practice. Today they generate a per device customized update that can be installed without user intervention. Even if they tomorrow enforce user intervention they still retain the capability to push a targeted update for a specific device on law enforcement/court order. The user has no way of telling what the update did.
It's very difficult, especially since they aren't open source. However, they could attain a state where to compromise a device requires the user accepting a malicious update, which would make the FBI's current request moot.
(although there's a whole separate set of legal attacks unexplored)
I think they could do all the stuff that makes it unhackable in hardware, and/or they could make it so updates aren't installed while the phone is locked.
It's not an attainable goal in practice. Today they generate a per device customized update that can be installed without user intervention. Even if they tomorrow enforce user intervention they still retain the capability to push a targeted update for a specific device on law enforcement/court order. The user has no way of telling what the update did.