It probably doesn't matter as long as you keep it consistent. If the counterfactual was N-1 instead of a replacement worker, then almost everyone would represent positive value, and you're still comparing multiple positive numbers. Yes, there exist people who are net negatives and the company would be better off without them, but not THAT many.

I think there are many employees who are net negatives (who create less value than their full cost to the company). Sometimes this relates to the employee; many times it relates to the circumstance the employee finds themself in. (In almost any spiraling downward company, there are many employees in this situation, through no particular fault of their own.)

There are far fewer who are gross negatives (who contribute negatively overall, before compensation and other costs).