Layoffs

I get the feeling that this is more-or-less how stack ranking actually tends to pan out.

I think the reasoning behind decimation was that it was supposed to be an equivalent bloodbath defeat. Historically, a rout typically resulted in about 10% casualties. Decimation reduced the calculus of a Roman soldier to 'stay in the fight, and risk a 10% chance of death' versus 'run, and risk a 10% chance of being killed at random by your tentmates'.