After the lessons learned from 9/11 I believe a hijacking attempt is a very remote probability. Could it happen, perhaps, but it would be much more difficult now. 9/11 was unique in that the hijackers had no demands of a political nature, all they wanted to do was take airplanes and use them as guided missiles, which they successfully accomplished with 3 of the 4 hijacked aircraft. The fourth aircraft crashed, but we know that the passengers and some crew fought back to overcome the hijackers, which is what will happen in all future hijacking attempts.
Where I think the threat lies today is just simple destruction of an aircraft while in flight. This would most easily be accomplished by a explosive device being loaded aboard the aircraft as baggage/cargo by an airport worker. TSA has steadfastly refused to deploy 100% screening of airport workers providing opportunity to introduce any manner of contraband.
Fighting last decades battle while new threats are evident seems a waste of resources. Cockpit barriers are a solution to an very unlikely problem while the real security threat goes unanswered.