Breeding billions of farm animals to feed ourselves surely isn't natural is it? I don't think this is a natural part of evolution. It is terrible for our environment, requires way more plant food and water than humans consuming those directly without having to feed all those animals we brought into the world forcefully and artificially (through insemination, forced breeding etc). We are also omnivores, not carnivores, and we can thrive without eating animals. Animals are sentient beings with a desire and need to live their lives in much the same manner as us, finding mates and raising their young to ensure their survival, enjoying fresh air, and exploring this wonderful planet. Who are we to deny them of that? Unlike carnivores such as wildcats and wolves, we do not need to kill animals to thrive. We have the capacity to understand this and find a way to live that seeks the least harm to others and to our planet. Why not do this?
Farm animals would not go extinct right away. It would take a long time if everyone were to go vegan. It wouldn't be overnight. And in fact, when farm animals begin to disappear, many wild animals that were wiped out due to farming and destruction of rainforests to make room for grazing and animal agriculture would likely return. Some farm animals were stolen from the wild and bred for thousands of years to become domesticated, such as pigs. There are still wild pigs and turkeys and so on. It isn't impossible to allow some to return to the wild, though yes most would die out. and what is so bad about no longer keeping pets and wild/farmed animals for food, entertainment etc. We can learn to live with the wild creatures around us and worry about our own population that is getting out of control.