Citadels have been around several years and are very effective. As noted, the crew can retreat to the citadel and can communicate and control the vessel from a secure location. The idea being that, if the rescuers know that all the crew is located in one safe locations, anyone running around elsewhere is a tango (bad guy). Problem is, citadels are quite expensive and the smaller shipping lines
can't afford them or they don't want to afford them.