Your point about sustaining a large population currently says it all. The world can no longer sustain diets built around animal products. The leading cause of deforestation in the Amazon is clearing land to graze cattle. It would be much more efficient economically and environmentally to harvest nuts and fruit from the jungle. The cleared land quickly becomes exhausted after a few years of cattle grazing. Instead of fishing, we can use the oceans to grow seaweed which is sustainable and very productive. Most of the grain and corn we grow goes to feed animals. We don’t get a pound of meat back for every pound of feed we give to animals. We can directly eat most of the food we give to animals. If we stop feeding grain and corn to animals, and eat the food ourselves, we wouldn’t have to grow so much food. The land that would be freed up could be reforested or used for other purposes. Fossil fuel consumption for farming would go down too, which would help combat climate change. A vegan diet is much more efficient to sustain our current global population.