There is only so much natural arable farmland on earth. If everyone is vegetarian, I'm not sure it would sustain current populations, without requiring deforest

There is only so much natural arable farmland on earth. If everyone is vegetarian, I'm not sure it would sustain current populations, without requiring deforestation/etc. There are many places in the world where people are sustained by animals like cows and goats which are able to eat things humans can't digest on their own.