There is no need to demonstrate that the intelligence is actively being misused. The simple fact that collecting it is unconstitutional and abusive is enough.
It's certainly enough to warrant an open political inquiry and changes of policy, as well as resignations from those politically responsible.
It's not at all obvious that all the NSA staff should just walk away from their jobs. If the NSA is actually protecting people, that would be just as irresponsible.
I think it's safe to say that they are not protecting people to any major degree. When pressed, the only justification they can come up with is "terrorism", which is a completely insignificant threat.
I think they are trying to protect people. It's just that they have been tasked with a pointless job. I don't think they're deliberately evil, they're just a massive overreaction to an irrelevant threat.
Basically, what do you think the immune system in somebody with a bee allergy is actually doing when that person gets stung by a bee? It's only trying to help, of course. But that doesn't change the fact that 1) a bee sting can be ignored and 2) the massive allergic reaction threatens the person's life.
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.
The NSA's efforts against Russia and China and other geopolitical enemies of the day are probably not much influenced by wholesale surveillance of American citizens. If the question is what the NSA as a whole is doing, the answer is, "a lot". I was concentrating on this one particular bit.
As far your IRA example, the US doesn't face anything like that, and shows no prospect of doing so. Even if it did, I'm not convinced that efforts like the NSA's would be very helpful in combating it, and given the example of the last 12 years, I'm quite sure that the US government's reactions as a whole would be far more damaging than the terrorism itself.
It's not at all obvious that all the NSA staff should just walk away from their jobs. If the NSA is actually protecting people, that would be just as irresponsible.