Ethical/moral objections aside, initiating the practice of human farming wouldn't be a logical or practical choice, as presumably farm-rearing humans would be just as energy-inefficient as farm-rearing livestock:
Animal protein production requires more than eight times as much fossil-fuel energy than production of plant protein while yielding animal protein that is only 1.4 times more nutritious for humans than the comparable amount of plant protein, according to the Cornell ecologist's analysis.
Killing and eating excess humans in the process of reducing the world's population to a sustainable level, on the other hand, might qualify as a logical use of resources.
When I envision a hypothetical future in which humans don't consume meat, I don't imagine everyone getting their protein from some kind of tank-grown super-tasty 'I Can't Believe It's Not McDonalds!' meat substitute . The meat-heavy diet of Western societies has no basis in evolutionary terms and I don't see why we should seek to perpetuate this relatively modern obsession and dietary imbalance. Contrary to what many meat eaters think, a vegan diet can be incredibly varied and tasty once you get used to cooking using a wider variety of herbs, spices and ingredients which aren't currently mainstream in Western cuisine. I personally find things like smoked tofu, coconut oil and milk and nuts like pistachios and cashews to be every bit as tasty as any meat product. The consumption of large quantities of red meat and animal-derived fats is cultural, not essential, and in terms of nuitrition not even especially desirable. The massive over-consumption of bovine dairy products is particularly nonsensical when more efficient, more nutritional alternatives exist.
Most everyone (except psychopaths) feels bad about suffering, and tells their friends the same, but they don't do much about it unless its close to their personal experience.
I'm not sure how much truth there is in this generalisation. Countless environmental activists, conservationists and humanitarian workers across the globe willingly give their time and energy to causes that have little or nothing to do with satisfying their own local needs or wants. Whilst they may not be in the majority, there are nevertheless a significant minority. I doubt many of them would be happy to be told they are only 'signalling altruism' to appear better in the eyes of their peers.
On the other hand, I suppose you could argue the case that such people have X-altruistic personalities and that perhaps that isn't a desirable quality in terms of creating a hypothetical perfect society.
What about a mentally disabled person, or other groups of humans who will never be capable of consciously entering into a 'moral agreement' with society? Should they also be considered 'outside the realm of morality'? What makes them different from an animal, other than anthropocentricism?
I do agree with you that many people have a romanticised idea of the natural world that probably has little to do with the reality; they appreciate the polished, TV-friendly aesthetics of nature documentaries without actually spending much time beyond their urban boundaries. I come at it from the perspective of someone who grew up in the countryside and love the feeling of being in wild places far more than in a city, so I suppose I have a different perspective. Personally I find busy cities really bring me down and leave me yearning for space and greenery.
Perhaps the ultimate rational position for the continued survival of humans and the reduction of suffering would be to have no animals at all and turn all available land mass over to trees for oxygen and the growing of crops for some kind of sustainably producible, nutritionally perfect food (perhaps a further developed version of the Soylent reference above), but pure rationality aside, don't we also value something that can't so easily be quantified about wild animals and the wild environment? I for one take great pleasure from the diversity of life exhibited on our planet. I would feel pretty depressed if I knew the future survival of life was predicated on such cold, unappealing utility alone.
So basically a chicken version of The Matrix?
Additionally, when there is a burden of evidence to suggest that nutrient-equivalent food sources can be produced in a more energy-efficient manner and with no direct suffering to animals (indirect suffering being, for example, the unavoidable death of insects in crop harvesting), I believe it is a rational choice to move towards those methods.