It isn't clear whether the 5S are newer are vulnerable to FBiOS
The secure enclave is basically a separate computer, which runs separate software. Can iOS updates deliver new software to the secure enclave? I don't know, but if so, and if updates to the secure enclave can be made in a DFU mode iOS update (which I doubt) then it would just be a different update.
However, Apple's upcoming change which blocks DFU updates entirely will block the All Writs Act angle. They'd be able to use that with iPhones they've already collected, but any collected with iOS 9.3 or later (or whatever version gets this) would be immune to that.
That's not to say congress couldn't pass a law that obligated Apple and other tech companies have some way to access data on their devices, effectively mandating some sort of back door, but that's a whole separate fight.
Biting the hand that feeds IT
