"There is an ongoing argument that one could easily support many more people by turning over grazing land to crops"
Yes, but I mentioned fruitarians, the people who think we should only eat the stuff that wants us to eat it, plus other wild (i.e. noncultivated) stuff. The "ongoing argument" works on the basis that all the insects, birds and small animals we would need to kill to live off cultivated crops don't matter as much as, say, pigs or cows. But intelligence is not a matter of size. Which has the greater right to exist, a cow or a cockatoo?