The Overton window's been that wide for a long time. Where do you think the term Soylent Green comes from? At some point in the past someone was talking about it. And where'd they get inspiration? At some point in the past before that, some tribe in Papua New Guinea or Africa or the Amazon rain forest was doing exactly what you're talking about. Cannibalism's not exactly a new idea.

The idea that humans could be used as livestock has been discussed and dismissed because we're horrible feedstock. We grow too slow and produce too little meat. Traditional livestock is much better. I've actually seen anthropology videos of cannibals saying exactly that to scientists.