This whole line of thought presupposes that the NSA would agree to do that for the FBI. It appears that the NSA is a DoD agency and part of the intel community and therefor does not work for the FBI. So, I see no basis for this assumption.
Are you telling me that if there's a real "national security" issue involving terrorists that the NSA would just say "naaah, go away, we're busy"? Not likely, considering they ultimately have the same boss, even if they are in different sections of the govt.
I don't think the FBI just wanders into the NSA and has them do stuff for them, but if it's truly a matter of national security there would be cooperation on the matter. What it tells us is that it a) is not really a matter of national security, or b) it's not an issue about this phone at all, that it's just a ploy to gain access in many other cases or c) both. I'm betting on c, both.
Many here say that the FBI is using this case to force a decision, but I think Apple is too. Apple is facing problems in Europe (France and the UK particularly). I think it might be that Apple wants to cement their legal position here in the US before taking on Europe.
Apple isn't the one that initiated this, it is the DoJ that took them to court to force their hand and force them to break the security on their phones. I also don't think what the courts here decide will make much of a difference in how the UK and France governments deal with apple.