Yes, but your point must be deconstructed. They had to approach their attack externally to find success. The airplane equivalency would be a missile attack against the plane. I am unaware of any security checkpoint at the airport that would address this issue.

Remember, you can be harmed anywhere (geographically speaking) a terrorist makes an attack. Do you think you are completely safe on an airplane? Once the notion that the airplane can be commandeered and projected as a weapon itself was dispelled by the ingrained psyche of a society of determined passengers crew, terrorists were forced to find another venue.

In addition, please remember that violence on an airplane is always a possibility regardless of security screening. Weapons can be created and modified easily from so-called "acceptable" items permitted on an airplane as has been detailed thoroughly in other postings. The only weapons of any concern should be firearms and explosives. Pocketknives, tweezers, and the like are rudimentary and should be accurately viewed as ineffective in a terrorist attack.

In the end, once pilots are armed and the cockpit door better fortified, a repeat of 9/11 will be virtually impossible much like it already is.