The question assumes that this is all negotiable, that there's some level at which we should be prepared to surrender fundamental freedoms. I claim that no such value exists.
Since the primary source of these threats seems to be from militant Islamicists, it would seem that we could get some measure of safety from banning Islam from America. How many thwarted attacks should it take for us to toss out the First Amendment?
It's difficult to prove that somebody is conspiring with a terrorist organization. Surely the need to prove a defendant guilty before a jury, is opening the door for terrorist attacks. How many thwarted attacks should we expect before we're willing to discard the Seventh Amendment? Oh, wait, we've already done that one in Guantanamo. Maybe we are on the way...
Surely you've noticed that there's a small but vocal minority that is heartily in favor of this, along with things like militarizing the border with Mexico, rounding up 1--12 million illegal aliens, and so on.
But it's actually my point that it's the ideals that matter. We can never say that the ends justify the means, because there's never actually an end: time just keeps rolling on.
So tell me: do you see me being unreasonable in a bad way? (This is a sincere question: what am I missing from my point of view?)
That's fine. The world needs idealists, but you have to understand when you don't get your way.
Is 50 threats stopped that wouldn't have been enough to justify the intrusion of privacy? Would 100? Maybe 1000?