Earlier quoted context omitted.
What makes you think the US Gov't can't break in and take this, once Apple has built it? They'll have to QA it, it'll be on at least a handful of machines---and the order requires it be network connected. At that point we should expect more world governments to get the key. Where does that end? Maybe with more companies having to demonstrate that they defend against their own subversion.
It can't be installed on any iOS device without Apple signing it at the time of installation. The software is useless without Apple's signing keys. Apple could sign it offline when installing to the device. The signature is customised to each device it's installed on, and has a nonce so it can't be saved.
The US legal system and education pertaining to it is largely composed of studying prior case law and its applications. Few cases (including this one) involve truly new decisions, and even those are usually novel applications of former decisions. That is what those who can see past their nose are concerned about. There is every probability that, in the near future, one or both of Apple or the judicial system will tire of the farce of one-phone-per-case firmware and request a new application of the law (because there would now be precedent).
To take any laywer-moderated statement (including Apple's) at face value belies credulity that does not become the HN audience. As others have pointed out, Comey's appeals to emotion and terrorism should be enough, but the idea that an FBI lawyer would advise pursuing this particular case in this particular manner without expecting to set precedent? Positively silly. This is what they do.