Exactly. A 9/11 style attack is also a single time event. Now that passengers know the outcome, they simply will not let it happen again. The only possible method would be to turn an existing pilot into a suicide killer.
The reinforced doors they introduced after 9/11 make this strategy particularly good. The passengers can't do anything if the pilot decides that he has a death wish.
An example of such would be the Germanwings Flight 9525 crash from May 2015, where the copilot locked the cockpit door after the pilot left for a bathroom break and then flew into a mountain.
