The NSA does not have a mandate from Congress: it was created by an executive order of President Truman in 1952 (https://www.nsa.gov/public_info/_files/truman/truman_memo.pd...). That order was classified for a very long time.

That's to say that you're starting from false premises. It is not the NSA's job to advance the security of the nation at all. Hence the need to split it up now. It's time to move the legal basis from executive order to something else, that something else publicly debated and mandated by the people.