The easy solution to this is to have the gov send Apple the phone. They break into themselves and then hand back the phone with the pass code turned off and whatever software they need to install to do it removed, leaving no trace of how they actually did it.
Win/Win
No software backdoor is created, the FBI gets its data and we all go on with our lives. Why are we spending so much time gnashing teeth over something that has a very simple solution to it?
Chain of custody. The guv would need to be able to prove, in a court of law, who touched it and when, for what purpose, etc. Handing it off to Apple in good faith would make it inadmissible as evidence in a trial.
You could argue that Apple could allow an official to be present while work is being done on the phone, but then Apple risks having it's (to-be-developed) method being viewed and/or captured by said official. Very risky.
Win/Win
No software backdoor is created, the FBI gets its data and we all go on with our lives. Why are we spending so much time gnashing teeth over something that has a very simple solution to it?