You could argue the complete opposite that this has anything to do with single man cockpits. Two pilots reduces the risk. At least that is what the authorities argued after the Germanwings disaster. As a now retired captain but with a solid background in work design I think this is much more about working conditions and job satisfaction. The pilot job has been continuously simplified and tasks automated. The focus from the manufacturers has been on safety and efficiency with a big upside for the paying public. The downside is that the job is much less interesting and ultimately requires less skill. If you pay people less and work them harder in uncomfortable shift work with boring repetitive routines where they don't feel they are adding much value, then some of them are going to struggle with their mental health.