You’re right that a lot of livestock feed is crop residues/byproducts humans don’t eat—but that doesn’t make beef “necessary” or erase the land/opportunity-cost problem. Globally, ~36% of crop calories go to animal feed and only ~12% of those feed calories come back as animal-product calories (Cassidy et al.). Livestock still consume ~1/3 of global cereal production (Mottet et al.). And in full-system LCAs that include grazing + feed land, meat/dairy provide ~18% of calories and ~37% of protein but use ~83% of farmland; cutting them can reduce farmland >75% while still feeding the world (Poore & Nemecek / Oxford). Plus, even if pasture isn’t croppable, it can be restored—land used for animal foods has a big carbon opportunity cost (Hayek et al.). Nutritionally, major dietetic bodies say well-planned vegetarian/vegan diets can be nutritionally adequate, with attention to nutrients like B12.