I don't know if I've ever heard of the NSA having any good effect on the USA, just always negative. Always leaks about how everything they're doing is illegal. There have still been terrorist attacks and mass shootings all around the country since it's inception. I agree with your point above that it's probably likely that these agencies are more so there to track and crush domestic and foreign dissent (and I think it's intentional, see COINTELPRO).

That money definitely could've been spent on infrastructure that benefited the actual citizens of this country and actually saved lives. Unfortunately a great part of American culture is pure jingoism and war propaganda, so nothing changes.