Hmmm, remember that BA BAC1-11 that lost a windscreen (and very nearly its Captain) over BHX about 15 years ago? The Captain was held by the cabin crew, who were able to gain instant access to the flight deck - any slower than that and he would have gone.
Frankly, the chances of something like this happening are infinitely higher than the chances of another September 11-style hijacking; and I think it's essential that the baby isn't thrown out with the bathwater on this issue.