When "doing something" is worse than "doing nothing", then yes we should do nothing. Off the top of my head, the things we've done because of 9/11:
* Create DHS
* Begin TSA security theater
* Start illegal NSA metadata collection program
* Afghanistan + Iraq ("not related", but had support from misinformed public) wars.
A good thing I can think of: more secure cockpit doors. On balance, yeah, I wish we had done nothing after 9/11. On the other hand it's not impossible for good actions as a result, but we have a pretty bad track record of it.
The importance of putting the _actual_ consequences in perspective is to let you at least consider that maybe we don't have to do anything sometimes. It's like the 80/20 rule or anything with diminishing marginal returns: to actually achieve flat out 100% safety and security is going to be an extremely expensive endeavor both in monetary terms and in how our society and ideals have to change.