Simply put, the FBI put themselves in the predicament they are in when they had a way to get all the information from the iPhone - but they chose to close that door permanently (or an IT scapegoat did) so they can continue to proliferate the lie that they need access to everyone's information. They are using this tragic act of terrorism as a lens to give themselves a halo in the public's eye, and we should all see it for what it is - cheap politicking to erode our rights.
Lets look back 10 years. If I had enciphered a letter that had all my thoughts and plans on it, the FBI would have to try and crack that. They couldn't force me to give up the password or key to decrypt it because that would fall under the 5th amendment protection . So the FBI would have to do a thing we like to call "their job" and detective work. Certainly that could be a lot of work on my part to encrypt just a few things, but possible for just a few important things... but the paper trail I would leave in everyday life would be immense - I cannot just drop off the radar. Not everything can be encrypted - bank transactions, purchases, public travel (e.g. stop light cameras or tailing a suspect), phone calls, etc. In the time before smartphones, the police had to rely on good detective work, and they can still do that today. This isn't a black and white fallacy
 that the Feds are arguing it is.
SCOTUS has shown the opinion that cell phones contain a vast amount of information and are not the same analogy as a bank teller opening up a safety deposit box for the police (with a warrant of course), or the police searching your car or home. :"Modern cell phones are not just another technological convenience. With all they contain and all they may reveal, they hold for many Americans “the privacies of life". The fact that technology now allows an individual to carry such information in his hand does not make the information any less worthy of the protection for which the Founders fought."
Alito even went so far as to say we shouldn't blindly apply pre-digital era laws... which is what the FBI is using by arguing the All Writs Act of 1789 . They are not demanding something simple, they are trying to compel a company to manufacture something that would unlock the doors of every house or every safety deposit box, and what's more is this weakness could be exploited by anybody, without a warrant, to access all that information. History is littered with reasons for why you shouldn't just blindly trust the government.Note:
We need government to keep a 'well' oiled and functioning society (for some definition of well) but government is made up of people and people are inherently broken. The system requires constant questioning and correcting to keep it on course.Update
: Other arguments I've heard elsewhere were 'What would you propose', which is sort of another fallacy arguing that I must have a counter proposal otherwise what I'm saying is invalid. Coming up with ideas is also saying that Apple should be conscripted to write this software - investing its own time, money, and engineering resources. I'm sure there is case law for this from a safe-cracker perspective, but that is a single person unlocking a single safe, one at a time. The government mandated a backdoor to locks already - TSA approved luggage - and those were all compromised by a single photograph that was posted online for less than an hour . I don't think there is a single, simple solution that will not end up harming Apple or John & Jane Q. Public.