I wonder if people pushing for stack ranking are justifying this in terms of natural selection. That is, even if the company is all top performers, periodically cutting bottom 1% or 5% or whatever is going to improve average performance even further over time.

This would make some sense if employees were like cells or fruit flies. But in most companies, employees are like organs. You can't improve a body by cutting out 5% of its organs and waiting for them to reappear. At best, you'll degrade the body's performance. At worst, the body will die.