I guess the issue with allowing voluntary departures is that you could lose numbers of engineers in vital functions. Also I imagine there tends to be a bias towards your most confident engineers leaving since they know they'll have the highest likelihood of easily finding a new role?

You can I've known a few co-workers who have. They had another job already lined up so when the boss came looking for heads to chop they volunteered. They got a very nice severance, a short vacation and a new gig and the boss was happy he didn't have to pick someone to fire.