There was also the fear of "when will the next plane hit?" The first plane hit and that was bad, but there was some sliver of doubt saying "maybe this was a horrible accident." Then the second plane hit. Then the third. Then the fourth crashed in the field. By this point, we were paranoid about all flights. Would any plane in the air suddenly veer off course and crash in random locations? Would they be taken over the minute we resumed flights?
This is what the terrorists want to inspire: fear. Fear of how much worse the situation could get. Don't get me wrong, they love the deaths they cause, but the fear really drives their cause.
I doubt that terrorists could pull off another 9-11 since the "standard hijacker protocol" has now changed. It used to be that you remained seated and quiet, your flight went to Cuba where the hijacker put on a big show, and then he gave himself up and everyone went free. Yes, your plans were horribly mangled but you were safe. Now it's, sit down and stay quiet and you're 100% guaranteed to die so you might as well fight back. That doesn't guarantee your survival but a chance at living's better than certain death.
However, terrorists could pull off another event with a 9-11-level of fear. Just have shooters/bombers stationed in various large airports at a busy time (e.g. Christmas) and hit them one after another. Flights would be canceled and people would be terrified to set foot in an airport again. And there is no security that could combat this (definitely nothing the TSA could do).
Of course, this doesn't happen regularly because it's hard to plan and execute this - especially when most of your followers are an ocean away from the targets.