I have a slightly different perspective.
The FBI asked Apple to help exploit a single iPhone 5C, Apple would complete the exploit and provide the unlocked device to the FBI. How that device was unlocked would remain within Apple.
Apple made a choice not to provide this exploit and therefore forced the FBI to look at alternative options, there is no reason for these options to be bound to a single device. The single device was only a requirement of the Apple court order that the FBI submitted.
Why would the FBI intentionally contact Apple, asking them to fix an exploit, that they wanted to leverage. Its like getting the keys to a safe and then asking the owner to change the lock. If Apple provided their support from the start then they'd still have the keys and they would only be giving the FBI an unlocked safe.