Let's be clear though: "cannot" for the government has always meant "legally not allowed to" and not "technically not allowed to". The FBI was technically able

Let's be clear though: "cannot" for the government has always meant "legally not allowed to" and not "technically not allowed to". The FBI was technically able to wiretap phones even after it was banned, for example, and you've already mentioned CALEA. It's possible there are technical software restrictions within NSA and not just "analysts are trained to comply with policy". I would like to think those technical backups are in-place, but I don't know if anyone from NSA has confirmed that.