It opens a whole new level of the conversation.
Consider this... the DOJ already has 12 more phones which they want Apple to unlock using the same software they want Apple to write for the San Bernardino case.
Federal prosecutors have asked Apple to unlock at least 12 phones. Are there really that many national security cases? Hell, the Manhattan DA has asked Apple to unlock a whopping 175 iPhones.
If Apple provides the lock breaking mechanism, where does it stop, what burden of proof will Federal prosecutors and the FBI will have to meet for the technology to be used? How do we know the government will honor its commitment to use it only for national security cases in light of the history they have of lobbying and pushing for backdoor access to encryption technology?
If Apple writes the software how can we be sure that governments like China, Russia or other stricter and/or more dictatorial governments won't compel the company to use the software to unlock phones and attack political dissidents, protesters, critics of the government with even stronger sanctions than they do now?
How many times has the government tried to do this before? For those of us old enough to remember that the US government used to class encryption at the same level of tanks and other restricted weapons see http://www.wikiwand.com/en/International_Traffic_in_Arms_Regulations.
PGP, one of the first publicly encryption software available for mass consumption, had to be re-released outside the US because of export and copyright restrictions. The US government investigated Phil Zimmerman, PGP creator, for "alleged aiding with ITAR violation."
There is ample precedent to be worried about governments' attempt at creating backdoors into encryption software. The slippery slope is very slippery indeed and one that is very hard to get out of