An extra serving would add more labor as the expensive part and not save anything. The way to avoid that would be to keep a fresh surplus in the kitchen which would get wasted anyway.
Essentially it would unfortunately amount to 'efficiency theater' without very large assumption changes - sodas for instance have free refills because it is already a massive profit margin due to the sticky price-point where everyone expects it to be a dollar something while the actual cost is measured in pennies. You would die of water intoxication before they would realize a loss from refills.
