Everyone knew, even in the immediate aftermath, that a copycat situation would never happen again, because passengers now knew to fight back and risk possible death vs certain death being used as living munitions inside a flying death tube.
Don't forget that in 2004 when traumatic memories of 9/11 were still raw and on everyone's daily consiousness, we had the Madrid bombings, the London bombings I think the following year, and then the Charlie Hebdo attack in Paris.
The world felt like it was rapidly becoming more unstable, and people feared that surprise terror attacks were the new normal. That kind of loss of confidence in security services can cause a rapid unravelling of the social fabric in ways that car accidents can't.
Nobody was talking about starting pogroms against people with brown skin because of car accidents.