For the iPhone 5C, they probably could design and build a special version of iOS, but it would require a lot of work. Not easily for any later iPhone. However, there is the potential that once built, it could be modified for others. The point is that Apple is not a party to this investigation, nor is there a prosecution involved; all potential defendants at this time are dead.
Complying with the court's order, absent any other compelling reason, using just an "All Writs" order, would put Apple's entire ecosystem at risk of destruction. No one is being prosecuted. It's a fishing expedition. There is no legal reason why Apple should be forced to do something against their interest and the interests of their customers, and their stockholders, merely because law enforcement would like to know what's in that iPhone.
“It’s a fishing expedition.”
Nope. They have a dead terrorist and over a dozen innocent civilians dead. And it is pretty apparent WITHOUT the phone that this guy had help over time. I don’t know about you, but I want the government to catch any more terrorists before they kill anyone else.
Cook’s problem, and the government’s problem, is that he built a sales pitch on anger and distrust of an untrustworthy government. Bad moves by both parties.
From a strictly legal point of view, I’m not sure the court order will stand (not sure if this is a SCOTUS ruling). From a PR standpoint, this doesn’t look good.