The nutrients that plants need to survive come from the decomposition of dead animals. Just because you can grow soybeans in one area doesn't mean you can grow strawberries in their place. Crops grown for animal feed are in most cases grown in areas that would not sustain the growth of all plants. I'm curious where your opinion that we could feed 6 billion ppl 10x over comes from. Can you link me a source so that I can become enlightened?