There was an oversupply of officers. At any particular point in time many of the officers were “on the bench” at half pay.
This made it easy to replace underperforming officers with those on the bench
This made it easy to replace underperforming officers with those on the bench
I suspect the RN was quite aware of the incentives facing captains, and could think a few moves ahead. And that captains understood their offers of commission weren't random - if you turned down a less-desirable one, you'd need friends in really high places for the RN to offer you anything better.