There have been numerous terror attacks in the US since 9/11. Any of them could have been directed towards airliners if security wasn't in place to deter them.
This is an interesting argument because it likely conforms with how the 50% of US citizens who actually think the TSA is doing some good view the situation. There are several lines of argument which suggest that this observation or the similar one that "there have been no more terrorist attacks on airliners since 2001" don't provide reason a to believe the TSA improves traveler's safety.
Perhaps most concretely there is the fact that there have been no terrorist attacks on the lines to enter TSA screening in the U.S. These lines often contain several hundred people who can't have been screened yet, because it is the line to be screened. This is a very attractive target for a terrorist attack, yet none has been performed in the US (it happened recently in Europe). That is likely because this type of attack is very rare and there just aren't that many people in the U.S. who want to commit that kind of terrorist act. It certainly can't be due to the TSA's procedures.
Secondly, there is the fact that the TSA's procedures only catch 4% of attempts to smuggle contraband through the checkpoints. Do we seriously believe that terrorists who are willing to die for their cause would be deterred by a 4% chance of being caught?
Thirdly, the TSA has never been able to provide any evidence that they have prevented a single attack.
Finally, and more abstractly, but perhaps most importantly, these are just extremely rare events so accurately reasoning about their causes or prevention takes a lot of data. The last time a commercial airliner leaving a US airport was certainly destroyed by a non-crew member prior to 2001 was in 1962. There was the explosion off Long Island which some theorized may have been a terrorist attack. Even if we count that, it would require 75 years from 2001 to be able to make a valid statistical argument that the rate of terrorist attacks had decreased since the institution of the TSA.
Given all of this data, what is the likelihood that the TSA's policies have prevented or will prevent a terrorist attack? I think quite low. From a policy perspective, that likelihood has to be balanced against the costs. Given the costs of the TSA, in dollars, other lives lost on the highways due to displacement to more dangerous forms of transportation, and the violation of innocent people's privacy, I think we have to ask -- is it worth it? Incredibly, the TSA has never performed a formal cost-benefit analysis of their policies.