I feel that the question of the door remaining open is of little importance as the security issues should be issued B E F O R E the pax arrive at the airport.
If a hijacker is armed and determined enough a lock door would be of little or no significance as the would have plenty of options on putting the aircraft in danger.
OTH a cockpit visit is a wonderful exprience for a great number of wanbes and some cockpit members...
Obviously there are plenty of issues that I ignore... so I stand to be corected.
Rwy in Sight