The problem is what do you do with that kind of intelligence, being so broad and ambiguous? Would you have grounded every last commercial airplane for a year if you found out they were planning to hijack planes? 3 years? indefinitely? This is why terrorism is so effective: it creates a panic that can cause more damage than an attack itself.
The best strategy is to go about your business unless you have HARD intel about the specifics of an attack, otherwise we end up shooting ourselves in the foot til we bleed to death