The first principle of designing security is to build defences that you can't breach yourself.
So Apple is being asked to develop an entirely new tool to do this, maybe they can, maybe they can't. If it was easy the FBI would have done it themselves.
Next we hear it's going to be a tool designed to only work on that specific phone. You can imagine how that'll go in testing.
Engineer, "hey boss I've written that program that only works on one phone",
Boss, "Does it work"
Engineer, "I guess so, we'll not know until we try, we'll have to plug the phone and see"
Boss, "ok"
Engineer, "Opps....."
So, what needs to happen is that a general purpose de-cryption tool is written and tested on hundreds of devices first before if goes anywhere near the phone in question.
Then if any evidence from this is used in court, the tool used to breach the phone would have to be given to the defense team, so they could verify the process and repeat the exercise.
You don't just get a cop going into court with a clear text print out and saying, "hey we found this on his phone, can't demonstrate how we got it though". Shits all over chain of custody of evidence and it being credible.
Also, the next time something happens, the police will use it again and again. It's nothing less than a general attack on presonal crypto