Those are good points. It would be interesting to know if pistons, rods, cranks, valves etc are the same whether for a 330 HP Cummins or a continuous duty rated version of the 5.9.
Take the very popular Cummins 5.9 rated at 330HP. The manufacturer states the engine should be run no more than one hour out of eight at full throttle or 2800 RPM. Total marine engine hours no more than 300 per year. As Ski notes, this is the case for marine propulsion or light truck duty.
OK, that same 5.9 block (?) can be used for continuous (prime design RPM) duty at 1800 RPM but limited to 120 to 180 HP dependent upon aspiration and fueling design.
Similar ratings for continuous duty gensets or water pumps(1800 RPM) are done by JD, Cat and Perkins for engines in this size range. I do know that for Perkins Sabre the engine innards are the same whether for continuous at 146 kW or lighter duty at 170 kW (up to 4000 hour per year) such as ferry or commercial fisherman service.
So for Baltic, running at hull speed or in his case 1300 to 1500 RPM, the engine will last "forever." Not so the engine hang ons though where marine age knows no % load.