Another point to consider: Even if the NSA had relevant data to the San Bernardino shooters, and even if the NSA was happy to share that information, that still

Another point to consider: Even if the NSA had relevant data to the San Bernardino shooters, and even if the NSA was happy to share that information, that still does not guarantee that the FBI would not move forward with the request to conscript Apple into doing their bidding. Prosecutors with agendas wait long and patiently for a case with the appropriate circumstances to make a request. If a prosecutor was waiting for a way in which to make sure that Apple wasn't able to eliminate the possibility of making requests that the government couldn't get access to, they would likely have to wait a long time for a better set of circumstances to make their case against. In this case - the victims are unsympathetic, and dead - the victims are unambiguously terrorists - the victims themselves do not own the phone - there's no concern of having to get things done secretly (e.g., there's no imminent attack that the government has to keep under wraps) A prosecutor who wants the All Writs Act to have the power they think it does would not allow this case to go unused, even if there was no material need for them to win the case.