Only about %25 of agricultural land is arable and viable for growing crops, the other %75 can only be grazed by ruminants like cattle. Just on that fact alone,

Only about %25 of agricultural land is arable and viable for growing crops, the other %75 can only be grazed by ruminants like cattle. Just on that fact alone, ruminant animals are never going away from the food supply until you could synthesize everything in a lab from some basic raw materials, and have that food actually be healthy to humans. That is long, long long away. Without cows, the prices of vegetables will go up a lot and put even more pressure on the remaining %25 of land. There is something to be said about growing animal feed on arable land to feed to animals directly although that could be used to make food that humans can digest directly instead. But you still need bio-available protein, and plants do really badly on that. If you actual compare meat to plant matter on what is actually digested and absorbed by humans to get the equivalent nutrition, you'll find that plants actually do not perform that well beyond carbs and some micro-nutrients on an efficiency basis. You still need fat, protein and the other micro-nutrients. Animals also convert indigestible agricultural waste products from plant growing into something humans can use, meat.