Here’s a hypothetical situation for you: Let’s say that suddenly, everyone, everywhere on Earth, decided to adhere to a strictly plant-based diet. No eggs, no meat, no animal products whatsoever. In that case, what exactly would happen to the billions of animals that are being raised for human consumption?
The answer comes down to supply and demand. When demand for something is at zero while supply is through the roof, the affected company will do everything in their power to cut costs. In all likelihood, that would involve culling all of the animals they’re raising for human consumption, because continuing to feed and slaughter them would be a waste of money. These companies already have processes in place (when mad cow disease necessitates culling thousands of cattle, for example), so they’ll just ramp that up. Getting rid of perhaps millions of tons of biomass will be tricky, but it can be done. In short order, all of the major meat processing companies would be forced to shut down or seriously downsize.
Agricultural animals sadly wouldn’t be able to survive in the wild anyway if they were all just released. Because they have no knowledge of (or experience with) the outside world, they’d quickly succumb to everything from disease and exposure to starvation and predators. Perhaps a small amount would be kept around for dog food production, some might be saved by animal rights activists or genetic specialists, and some may be donated to zoos, but in a nutshell, the whole world going vegan means the nearly-immediate die-off of billions of animals (who were all slated to die anyway).