I,too,believe that our flight controls are our best first-line defence.Not that it will deter the hijackers in the long run,but it will give the flight crew a chance to deploy masks and start de-pressurizing,thereby immobilizing the bandits.
In a previous job flying light twins on air taxi,small freight,etc.,we often had fun on empty ferry flights by sitting in the cabin,one at a time of course,and performing 0 G maneouvres.As soon as you leave the floor,your body starts to decellerate and you move towards the rear of the cabin.One might be able to "deliver" the bandit to the pax.You know exactly when you reach 0 G,determining 2 G or more might cause more problems.
May we want CO2 extinguishers back?They can really stun people with their extremely cold temperatures.They can also make excellent sledgehammers.
As for a "hermetically sealed" flight deck,well,I'm not so sure.A door which will delay entry,maybe,but what about pilot incapacitation,etc.?