Second, it's unclear how easy it would be to create firmware that only works on a specific device. What if there's a bug, or what if someone finds a way to spoof the device ID that would probably be checked? It could very well end up being a backdoor for all iPhones after all (or, at least, for all iPhones without a Secure Enclave).
This EFF article has a couple more points[1].
[1]: https://www.eff.org/deeplinks/2016/02/technical-perspective-...
Let's say that Apple crafted an update that does what the FBI wants. That in itself wouldn't put other iPhones at risk, as long as a signed copy of that malicious update didn't leave Apple's custody. The FBI hasn't even asked for that. They say that Apple could do all the work in house. Furthermore, it's unlikely that this malicious update would work on newer iPhones.
But the precedent would be established.
And once the software leaks, which will eventually happen, everyone will be able to do it in every situation.
Moreover, every other government will ask for it, and not all governments meet the same moral standards.
once this is created, apple would pressed HARD by all authorities to re-use it for all iphones captured. these days we know damn too well that people in CIA, aNSA etc don't hold any reasonable moral values when it comes to privacy of about anybody on this planet.
plus it could be taken from this specific phone and very probably copied anywhere, without any apple approval or court order.
if government was playing mr nice guy till now, there would be at least some hope and faith. currently, there is simply none.
Once that update exists, it can be installed on any device because it has been signed by Apple.