Can the average sailor ever truly become an expert on all systems, while maintaining a full level of competency in their own fields?
I qualified submarines, something I'm reasonably proud of.
Does this mean that I could trace the entire hydraulic system? No, but I knew where the key valves were and how to isolate it for damage control. Does this mean that I can fire a torpedo? No, but I knew the rough principles behind the system and I could use that to guide my use of a manual to load and fire one. Could I send off a radio message? Not without referencing a manual. Steer the ship? That I could do, actually, but I'd be damned if I'd ever be as good of a helmsman/planesman as a coner. Use the sonar system? By the book only, I'd have no idea what sounds meant what. Launch a SLBM? Hah, I don't think I could manage that with the books handy.
On the other hand, it meant that I knew the roles of each watchstation, each system, a rough idea of their function and their criticality to ship operations and safety. How to fight a casualty, where to go to fight the casualty, etc. Where a given panel was, where and how far each firehose could run, where the extinguishers were located, all of that sort of thing. I don't think anyone could truly master every single watchstation and system onboard and maintain proficiency for their actual duties.