Voluntary retirement/resignation sounds like it could only ever be a bad idea, even financially.

The people who choose to leave are the ones who know they'll land on their feet and find another place. ie: competent, experienced, in-demand. Possibly being head-hunted.

The ones you can't easily bribe to leave are the ones that don't expect to find alternative employment in a reasonable time frame (or rate). So... inexperienced, under qualified, or low production.