I take it these suspects didn't have any backups of their phone? It clearly cannot be the case that the backup can only be decrypted by the original device, since the entire point of the backup is to be able to restore it to a different device.
But that doesn't meet the FBI's actual needs. The FBI's ACTUAL needs are to have a case with a lot of public sympathy in which they can force a major tech company to very publicly comply with their order to add a backdoor to a phone (without calling it that) in order to influence the legal and legislative systems (and perhaps public opinion, if the FBI even cares about that).
http://www.nytimes.com/2016/02/19/opinion/why-apple-is-right...
Source: http://www.politico.com/story/2016/02/apple-iphone-privacy-j...
On the other hand, if the FBI had some lead time, all of the above could be circumvented without Apple's cooperation.