I'm just an engineer. But I'll design you a cockpit interface which can't be penetrated without serious power tools, and which will function as required for the de-pressurization case as well as for other HVAC cases. No charge. Will the airlines be prepared to pay the cost in weight and revenue loss? We shall see, I guess.
As with all the other suggestions made, there is no one silver bullet which will fix all these problems. What seems to be significant is to look at a whole spectrum of options. Not all of them have to be deployed, all the time. Not all of them will work in all cases. Some, at least, will involve some terrible decisions on the part of flight-deck crew, and those of you that do that are going to have to start looking within yourselves to see whether you are ready to do those things.
If someone starts killing cabin crew or passengers to try and make you let them onto the flight deck, there is a good chance now that they want your airplane for keeps, and not just for you to take them to Cuba.
When you push back, you take the lives of every piece of SLF in your hands. That's enough responsibility for most people, right there.
Now, when some wild-eyed maniac starts cutting up the passengers or the cabin crew becasue you won't open the door, there's a good chance that he's not only planning to kill everyone aboard (sooner or later) but also to kill as many people on the ground as he can get to. Those of you forward of the cockpit door are the only hope then, not only for everyone behind you but for large numbers of people on the ground as well. You may well have to decide that the people behind you are going to have to take their chances in order to avert much greater loss of life below.
Some of you may be ready for that terrible calculus. Others may not. But I think that you should at least consider giving the tools to make that work to those who are ready to use them, and that means a cockpit bulkhead that will keep you alive and capable of putting the a/c on the ground - any ground - where it can't hurt anyone else.
Of course security at US airports needs to be seriously upgraded from the present joke status. Of course we need tools to identify and extract people who would do this sort of thing. Of course we need to explore ways to prevent a/c from being used in these ways, even if they are taken over by skilled pilots who welcome death. But I submit that giving the flight crew the option of keeping the a/c out of the hands of such aggressors is just another part of this whole spectrum of prevention.
llater,
llamas