I understand the point you're making, I just don't think it's obvious that the NSA is unimportant. As well as detecting terrorism, they may well be doing a lot of other things to protect the us against then machinations of others e.g. Russia and china.

Also, don't underestimate terrorism. Over decades, the IRA did a lot to disrupt the UK, including killing cabinet ministers, and two nearly successful assassination attempts against the prime minister. Ultimately the UK conceded and the new Irish government contained ministers who had been leaders of the terrorist movement.