The weight increase resulting from a fully sealed cockpit with its own access door does take away the threat to pax and cabin crew.
But does it also increase safety levels? It takes out an emergency exit. It also prevents the pilots from going into the cabin in case of technical problems: it will be impossible to look, listen or otherwise observe what's going on when the a/c does not work as adertised.
Events that justify an access door have been a lot more common throughout the years.