iPhones already cannot be downgraded, they can only install OS versions signed by apple during the install time. (search SHSH blobs) They also can't run unsigned IPA files (apps). Not sure if they have a physical fuse, but it's not much different.

The significant difference is that if it were placed into DFU mode and connected to an appropriate device that had access to appropriately signed things, it could be "unbricked" without replacing the mainboard.