You don't have to resort to complicated cryptography: anything the FBI can decrypt, anyone who knows what the FBI knows can also decrypt: is is prima facie impossible to have a decryption algorithm that only works when accompanied by a valid warrant.

So that's it: there's actually no way to allow the FBI to decrypt an iPhones without also assuming hackers will (perhaps after a delay before a leak/weakness is discovered), you only get to choose "hackable" or "not hackable", there's no "hackable only by the honorable American government"

Also, it's already very hard to make things "not hackable" without deliberately introducing a certain way to hack it.