What Is the Opportunity Cost of Including a Zero-Fee Transaction in a Full Block?

The opportunity cost of including a zero-fee transaction in a full block is the lost revenue from the highest fee-paying transaction that was excluded to make room for the zero-fee one. Since block space is a scarce, finite resource, every byte occupied by a zero-fee transaction could have been occupied by a transaction paying a competitive fee.

A rational miner, seeking to maximize profit, will never incur this opportunity cost, as it directly reduces their total block reward.