It's not just about that one phone. The Atlantic already reported that court documents show it's about at least 12 phones. Then after that, surely the floodgates would be opened.
If Apple complies with this, they tacitly set a precedent. Then, every agency (NSA, CIA, every other government in the world and all their agencies, etc.) will come clamoring to have things unlocked.
So Apple will then have to leave backdoors in their phones in order to continue being able to comply ... they simply can't keep making unbreakable phones, then wasting tons of time finding a way to break it, over and over again.
If they leave a backdoor then hackers will find it and nobody's phones will be safe. You'll plug your iPhone in a charger in a Chinese hotel and it will be owned. Etc. Defense contractors' phones will get owned, China will steal yet more secrets; terrorists will hack airline pilots phones and be able to impersonate them and hijack planes; hackers will get into nuclear facilities through compromised devices.
We just can't have insecure phones. This is where I store my heartbeat, my sleep cycles, my wallet, my most personal notes, logins to all my social media accounts that could be used to ruin my reputation; I would not dare trust it if I knew there was even a chance of it having a back door.
The price of having privacy in society is the risk that criminals or terrorists may use that privacy to conceal their activities. I'm willing to pay that price; it's the price of freedom.
Those brave soldiers who fight to protect our freedom are fighting to protect this: our right to privacy and individuality and freedom from constant surveillance.
If we give up that, then the terrorists have already won.
