Of course they replace people if they are doing something more efficiently than people, but what scares me is the law of unintended consequences (there always are some). For example lets take a bot which picks content which will become popular. The bot gets really good at it, it earns good curation rewards and people don't so much, therefore people stop doing it so much; the normal evolution of collective thought and social taste therefore becomes subject to some serious inertia, because the bot is always picking what we would have liked yesterday rather than what we like to day, and because that then gets pushed to the fore and yesterday's thinking drowns out any new thoughts we may have had today. What's more, the bot misses edge-cases which it doesn't know about, so all exotic, unusual or unique thoughts which would have been surfaced by human curators end up being buried in a sea of innocuous and slightly stale memes.