That's probably not the best way to frame it, but it's important to make people aware that they can't work 60 hours at the same quality of the first 30.

Even if they could, they'll either burnout, get promoted or leave within 24 months and then you'll need to backfill with multiple people.

Even as an evil manager, you're still better off stopping people from overworking, as you'll get more work overall from them at a sustainable pace.