To hear US law enforcement respond, you'd think Apple's move heralded an unstoppable crime wave. See, the FBI had been using that vulnerability to get into people's iPhones. In the words of cyberlaw professor Orin Kerr, "How is the public interest served by a policy that only thwarts lawful search warrants?"
Ah, but that's the thing: You can't build a backdoor that only the good guys can walk through. Encryption protects against cybercriminals, industrial competitors, the Chinese secret police and the FBI. You're either vulnerable to eavesdropping by any of them, or you're secure from eavesdropping from all of them.
Backdoor access built for the good guys is routinely used by the bad guys. In 2005, some unknown group surreptitiously used the lawful-intercept capabilities built into the Greek cell phone system. The same thing happened in Italy in 2006.
In 2010, Chinese hackers subverted an intercept system Google had put into Gmail to comply with US government surveillance requests. Back doors in our cell phone system are currently being exploited by the FBI and unknown others.