That would arguably still be better for the planet. Plant agriculture is lower-impact and poses far less threat to biodiversity and the human species via diseas

That would arguably still be better for the planet. Plant agriculture is lower-impact and poses far less threat to biodiversity and the human species via disease vectors. Take for example that lettuce is a leading cause of food-born illness. It's actually because there's poop from animals on it, and it almost always comes from agricultural animals with the intent of it functioning as fertilizer. Without that factor, plants would be far safer. There isn't much about animal ag that makes good sense apart from "it tastes good".