You're conflating two things (probably intentionally, but I'll go with it). If you are firing someone without replacing (a layoff), then presumably, that person cost more than the value they provided, and so laying them off should improve the value of the business (after incurring the costs the parent post mentioned).

If you are firing someone to replace them with someone else, you are incurring a lot of cost (hiring is time consuming, difficult, risky, and requires a ramp-up time before the new hire is productive), and hoping that the long term benefits outweigh that cost.