The water ban came later for a different reason, namely the "Liquid bomb" plot, which involved bringing liquid bombs onto planes and detonating them mid-flight. I'm not saying the ban was an especially good reaction, but cockpit doors wouldn't have helped. You don't need to be in the cockpit to bring a plane down with a bomb.
Right but the original context was "another 9-11" that would lead to "decades of war". We have clear examples, right or wrong, where bringing down a single plane would not result in that response by a nation state.
One of the reasons we have not seen another airplane based terror event, is not because of security, but because it simply would not invoke the response the terrorist desire. They want a nation to over react, to bomb things, to over all make the lives of the people they purport to be fighting for "freedom" on behalf of worse... This keeps them in power, and aids in recruitment.
Taking down a single plane does not do that, using that plane as a missile did, which door locks prevents.
Making me buy over priced water == Security theater that does nothing for safety
We do not need to be illogical about it, extreme government over reach should not be excused when a simple door lock was all that is needed