Maybe humans are not the best electricity generator, but slavery is far from obsolete. In fact, there are more slaves today than ever before in history although it's difficult to define exactly what a slave is (https://history.stackexchange.com/questions/41684/is-it-true...). Humans may not be so efficient but they sure are cheap, in some places as little as $90 (https://socialinnovation.blog.jbs.cam.ac.uk/2015/03/17/moder...). The sole reason slavery is not as widespread at least in the developed world is because of laws caused by some people having a conscious. When profit is in question unfortunately people will go very very far to minimize cost.