You are assuming they are being terminated after a correct assessment if their value. Good employees still get fired. Managers can be wrong, or play office politics.

You say "obvious", but in the real world, the lines of net positive or negative have more inputs than that. Adding an algorithm to the mix that may or may not be operating correctly is not a human way to operate.

In civilization, we make affordances for people that go against the razors edge of pure efficiency.