This is not a simple issue, cut and dried. In the first place, one wonders why the government ever made public its inability to crack the encryption of the phone. The wiser course of action would have been to have let the terrorists believe the phone was yielding mounds of important information. Maybe ISIS would have figured it was a bluff; maybe not; but better to plant seeds of doubt in your enemy's mind than to let him know his secrets are secure. Or perhaps they should not even have mentioned the phone - again, it can throw an enemy for a loop if he doesn't know what you do know about his plans.
Second, the government should have asked for Apple's help quietly and behind the scenes. Perhaps turned the phone over to them, asked them to open it and then taken it back once the task was accomplished. This would have removed questions of public pressure and might have allowed the code to be broken, which is all the government should have wanted, anyway; to find out if the terrorist had been in contact with others and had been involved in any other potential plots. The constitutional questions are important, perhaps central, but are not the immediate concern here. Protecting Americans from future attacks is. Better to do that than to be distracted by weightier but less immediate matters.
While encryption can and does abet criminal or terrorist behavior, most encryption is done for legitimate reasons of security and data protection. It is true that breaking into a phone's encryption might save lives and prevent a terrorist attack, but it is a power that could easily be misused if it fell into the wrong hands.
The government may be able to make a good case against Apple, but that case is weakened when one recalls that the San Bernardino terrorist had freely traveled to the Middle East for extended periods of time, and while overseas had married under circumstances that should have raised red flags for the immigration authorities. Furthermore, his bride submitted false information on her entry documents and the government did nothing about it - wasn't even concerned. Even a brief review would have determined that there was potential danger about both the man and the woman. In other words, the authorities had ample opportunities to curtail those terrorists, to keep them out of this country, to deport them, to detain them, to prosecute them, and did nothing. Now, they expect Apple to make everything better, and are prepared to blame the company if it doesn't cooperate. That's unfair.
The FBI Director has testified that his agency made things worse by supervising a change of passwords. This was done in the interests of security but it ended up making their task more difficult. Had this been handled better in the first place we might not be at this point now. It would be a good thing if the authorities were able to read the contents of the phone, but we don't know if there is anything worthwhile on it to be found. The present cohort of terrorists seem not to be directed by headquarters, but more likely are inspired by words and deeds to take individual action rather than following orders from some mastermind.
Apple has cooperated with law enforcement many times in the past and probably will continue to cooperate in other types of cases. Surrendering to government the power to remove encryption could be a step on the road to tyranny; it might in this case or another case unlock the plans of a terrorist cell. Once freedoms and protections are lost, they can never be regained, and once lives are lost, they never can be regained.
As we said, this is not a simple issue, cut and dried.