Claiming the TSA agents at the airport are the first line of defense is idiotic thinking. If the bad guys get that far then everything else has failed. They are the last line, and not a very good one. (Bad guys can try multiple times, and use multiple people - they just need one to get through while the TSA needs to stop every single one.) The other thing/something else is good old fashioned police work and intelligence to discover/prevent the plots before they reach the stage of people at the airport.

The TSA could never be perfect - consider the example of prisons where there are essentially no rights, lots of searching, controlled access etc. Things still get through.

And even if they stopped everything getting on planes, they are still reacting to a tactic which has repeatedly been the problem. The bad guys can just blow up queues at the airport, or a mall, or a stadium, or a highway. There is no shortage of targets or methods that will instil fear. (Good old fashioned police work and intelligence should be tactic agnostic.) The reality is that not that many people actually want to be bad guys, and a US policy of not giving people reasons would be a good one.