Sure it is possible they decided to back off, either because they thought they would lose or couldn't risk a loss even if they felt they had a 99% chance of success.
However, given that it was an older phone that predates the secure enclave, it would be a lot easier to get in than one of the newer ones. And even the newer ones aren't necessarily immune, but now that Apple knows they need to defend against potentially being forced to hack their own phones they can design against that possibility. I suspect that iOS 10 will remove the ability to install an OS in DFU mode without providing the password or otherwise authenticating yourself or the connected PC to the phone. That would completely block the angle that the FBI was asking the court to force Apple to attempt.