The sad thing is that the US intelligence community apparently notified the French spooks about the possibility of an impending ISIS attack(s) (which seems ambiguous, after all, an attack could occur anywhere) and a few of the attackers were on fundamentalist watch lists, and yet the attacks still happened. Yet no major media outlet has (at least that I have seen) identified this as an example of a failure of the US/EU ICs. Glenn Greenwald was on CNN yesterday throwing CNN's anchor under the bus for allowing all these major IC talking heads to come on and blame snowden, talk up dragnet, etc. - it was pretty interesting.
Terrorist attacks, which are black swan events, are not preventable. So if I have to deal with them, I would rather deal with them without the dragnet than with it. I also cant think of a single example of the NSA/CIA preventing a major terrorist attack in the US (at lets face it, if they had one, they would have used this as a shining example by now). All of the attacks that were thwarted were just by luck: shoe bomber, underwear bomber, time square bomber, etc.