The problem here is that we are rationalising a response to people whose values seem completely irrational to us, so we struggle to equate our thinking to theirs.
If the flight deck door is locked and hijackers are running free, what other tactics could they try? Popping the doors or hatches and trying to cause damage by depressurisation (if this is not feasible, forgive my lack of knowledge.)
It seems a reasonable response to lock the f/d door under the circumstances (will make the slf feel better if nothing else and their cash is vital at the moment), but it is treating a symptom.
Increasing security on the ground is also a reasonable response, but I guess what is needed is a full review of how ops work and some thinking outside the box to deliver an end-to-end solution.
I wish i was clever enough to come up with an answer, but I am not.
As a regular traveller, I just hope that the authorities and carriers seriously address the new situation we find ourselves in as a matter of urgency.