You're missing the cost of talent acquisition. Assuming the company requires skill A, the actual cost is more like $x+$y+z where $z is the total cost of talent

You're missing the cost of talent acquisition. Assuming the company requires skill A, the actual cost is more like $x+$y+z where $z is the total cost of talent acquisition, including time spent by the hiring manager and recruiting team, disruption to the productivity of whomever sits on the interview panel, and the ($x+$y) for however long of a period it takes for a new employee to get up to speed. The more frequent your turnover, the higher $z becomes.