While animals can eat waste from crops grown for humans, that’s a minority of their feed. Almost all new corn and soy cropland is cut to meet growing animal agriculture demand.
How does eliminating animal fat and protein cause health and nutrition problems? Humans are perfectly capable of living on vegan diets at all stages of life.
I do! But I also value animals, and consuming their dead corpses is completely unnecessary and on top of that is wasteful, hurting humans in the process.