Some consumers may choose veganism, or a pescatarian diet, but meat, eggs and milk, offer crucial sources of much-needed nutrients which cannot easily be obtained from plant-based foods, a new report issued on Tuesday by the UN’s Food and Agriculture Organization says.
We don't actually know exactly why people started farming chickens. It was almost definitely in Southeast Asia, but the reasons that people participated in any kind of agriculture have been varied snd changed over time. Eventually it became simply a form of food, and one that synergized with cereal production, but raising chickens predates the existence of having any significant amount of agricultural waste to feed them.
Your idea of having "ethical" decapitated chickens makes no sense. Nobody does this and it would contradict the labor saved by letting chickens deal with ag waste. It's also just plain infeasible because cutting at exactly the brain stem isn't easy.
In a world of industrialized agriculture, our ability to produce enough food or nutritious food is not the problem. Ag waste can be recycled straight back into ag or used in other ways abd is unnecessary for food production except when poverty has been forced onto people artificially, such as through imperialism. Groups like the FAO like to talk about malnutrition in countries in Africa while ignoring the elephant in the room: the empire has forced IMF "restructuring" on them, has undercut domestic food production, and turned their economies into extraction industry and perpetual poverty. In this status quo of deprivation, liberals squabble over whether having chickens or a cow is "the fix". Meanwhile, industrialised ag can produce more and better food, and cheaper, but it is deliberately made unavailable by global capitalism.
Anti-veganism is not particularly well-grounded in a material analysis. It's usually just reactionary excuse-making and recycles the same kinds of self-serving talking points I've heard from "leftists" that work for defense contractors.