grand slam has got it almost 100% right. The terrorist will find a new way.
While I appreciate that this is an aviation forum, don't ignore the bigger picture. The terrorists will find some completely different vulnerability to attack. Close off aviation completely -- if such a thing were possible -- and they'll blow up the tube or buses. Oops, sorry, that's been done. And we can't actually do a damned thing about a possible repetition, however the politicians and their "expert advisors" huff and puff.
Pick another target: how about a few major hotels in London? That would do the trick.
My point is that we fixate on the least likely or the flawed: like flight deck doors. We'd been through all that in the past and relaxed it because the threat changed. Come 9/11, and we pretend that we can eliminate the threat with armoured doors (again). Which are opened by CC to take the flight crew coffee and meals.
You have to keep the bandits off the planes, it's already too late when they're aboard.
I could go on, but I'd bore you to no effect.