* I guess I figured if they simply can't do it why not say "it's impossible by design," rather than argue the principle?*

Because the larger principle is what's really at play here. Whether Apple can do what the FBI asks, or not, is irrelevant. What is relevant is that what the FBI asks is bad. Even if Apple can unlock this phone because of shortcomings of the 5C (versus later model), it sets a bad precedent for later, especially if Apple truly cannot unlock the 6s and beyond ("you could do it on the 5, why not an iPhone 7?").

I'm no lawyer, but I'm of the opinion that this is a legal crowbar for later cases, and the Feds are using a tragic incident to drum up support ("you don't support terrorists, do you?")

Maybe it's impossible by design in current iPhones. But this is an older one, not so secure.

As Tim Cook says, it would be a bad precedent. And obviously bad PR for Apple to admit vulnerability.

It is, of course, possible; any iPhone can be forced to enter the restore mode or the DFU mode (don't remember which one) and be erased/reflashed via iTunes.

It won't get unlocked by that, because as soon as the new iOS takes over, it will detect the activation lock and require the Apple ID password to be provided. Also, going through iTunes erases all the data.

But presumably a custom DFU update can only replace the OS without replacing the data. The iPhone doesn't try to protect against valid updates, and will happily run anything signed by Apple; the locked/unlocked state is only about the encrypted user data on the device.

Thanks. I was thinking in FDE terms. So maybe what the FBI wants is doable.