The training burden behind a useful naval reserve

Would every navy benefit from expanding ready reserves to address trained-personnel shortages?

Yes, if it can keep those reservists trained; modern simulation makes that increasingly affordable.

Reserve manpower is broadly valuable, but nominal numbers do not automatically create a ready fleet. The central problem is maintaining technical skills and team proficiency between periods of active service. Interwar navies had to devote scarce ships, instructors, and operating funds to that task, which limited how large a genuinely useful reserve could become. Modern shore-based simulators make economical skill maintenance more plausible, which makes a ready reserve more feasible for present-day forces than for fleets in 1939.