There is nothing wrong with organic agriculture. We need to use manure, and in some cases we may have to start using sewage by-products in order to help grow the food we need, but the reality is those products are usually in the wrong place. They're not really near the major crop-producing areas. Their livestock tends to be aside from the major crop areas. It's very expensive to move very low-nutrient-content products long distances, and the scale of agriculture that goes on in places like western Canada, Brazil, and Russia just doesn't lend itself to that kind of small-scale solution.
There is nothing wrong with using manure or compost to grow crops. The problem is you can't feed 9.6 billion people doing it, and that is the big problem. If you have a farm where you have a few goats and a few chickens, all the manure stays there, you feed yourself, and you don't send any food away, then you can have a sustainable agricultural system. But if you have to take that grain and ship it around the world, you have to find a way to replace all the nutrients that leave the country to feed people elsewhere.