I won't rule it out but I think it would be way more difficult to pull of a 9/11 style attack in a post 9/11 world. Even ignoring security theatre and other changes like reinforced cockpit doors I think it's unlikely for hijackings to be as successful in a world where people know an airliner can be a missile and not just a hostage situation.
Unless the pilots themselves decide to do a 9/11...
Some even have dual Canadian Chinese citizenship so they can play on the Chinese national team in the Olympics