After the horse has bolted.
As a none pilot please permit me to put forward a theory? Before 9/11 Locking doors were a very good idea but from that point the parameters changed. Security measures were massively upgraded to prevent multiple terrorists getting aboard. From that point it was better to have the security doors removed. If a lone terrorist or someone unstable mentally manages to get aboard then the whole plane can stop the situation being fatal. Is it not a case of reacting to something that happened in the past with a solution that no longer perfectly fits the changed parameters of the present? Keep the massive airline security but remove the doors so that a lone threat cannot become the sole person in charge of the controls?