Ben Thompson has a great summation of the dispute between Apple and the FBI:
A master key, contrary to conventional wisdom, is not guessable, but it can be stolen; worse, if it is stolen, no one would ever know. It would be a silent failure allowing whoever captured it to break into any device secured by the algorithm in question without those relying on it knowing anything was amiss. I can’t stress enough what a problem this is: World War II, especially in the Pacific, turned on this sort of silent cryptographic failure. And, given the sheer number of law enforcement officials that would want their hands on this key, it landing in the wrong hands would be a matter of when, not if.
Thompson worries that Cook has picked the wrong battle–i.e., that it shouldn’t be about the circumvention on this particular phone. That’s in part because the phone in question–an iPhone 5c–lacks security protections present in the 5s and later; so the workaround the FBI is requesting isn’t even possible on newer models.
But I’m sure there are also millions of 5c and earlier iPhones out there that would potentially be subject to such procedures, which is likely one reason that Apple and Cook have taken a stand here: it’s aiming to protect all of its customers, not just some of them.