This is moving the goalpost, though. The discussion was about whether or not the replacibility of generalized human labor is comparable to the replacibility of

This is moving the goalpost, though. The discussion was about whether or not the replacibility of generalized human labor is comparable to the replacibility of horse labor. No doubt there can be reasonable disagreement, but we can't compare the adaptation to specialized function replacement with generalized function replacement. The point of the analogy is that it (generlized labor) is novel in human populations, but not for horses.