Lost opportunity is a real, actual, bona-fide cost. But I'd agree with you if you argued that the cost is small, per-copier, and that many copiers would never choose to buy the material in the future anyway.
But when the person who found value, and would have been happy to pay, sees others paying nothing, that person will then also pay nothing. That's a real lost opportunity. And worse, it extrapolates out, obviously, to less compensation to artists, inventors, scientists, actors, etc., which leads in my opinion to a less fun life.