Actually, most of the "failed terrorist attacks" actually succeeded in making us scared. The "failed" shoe bomber means 800 million people annually now need to take off their shoes every time they go through security, taking a cumulative 760 man-years of time (assuming 30 seconds for on and off), of monetary value $67 million if you assume $10/hour value for the average person's time. The "failed" underwear bomber, now means 800 million domestic airline passengers annually need to be xray-screened, and costing us even more in (useless) machines, all for a bomb that probably cost $5.
I think we're wasting way more time and energy reacting to their past attempts than they are thinking of new ways to try to hurt us.
*800 million from http://www.numberof.net/number-of-airline-passengers-per-year/