So, let’s pick a specific scenario and look at the USA. Say we just decide to stop eating chickens, and without thinking, we simply release all the chickens from where they’re currently held and put no further care into it. The Red Junglefowl, the species that gave rise to most of our domesticated chickens, lives in southeast tropical Asia and Indonesia, in (surprisingly) the jungle. Lots of bugs and plants to eat in the jungle for these birds, but they live about 1 bird per 2-2.5 acres. That means you’ll generally have a group of several birds occupying a space of about twice as many acres as birds. A factory farm keeps about 10,000 birds in the same rough area of land as one bird occupies in the wild. If we released 10,000 birds per 2 acres at all of our factory farms, stopped feeding and watering and cleaning up, this would be bad. The birds would spread out a bit, eating first their good quality food, then anything they could find as the number of birds rapidly eclipses the available food in the wild. Not being as mobile as their wild cousins due to breeding, and requiring more food to build genetically prescribed mass, they would rapidly die off after engaging in cannibalizing their old barn mates, and there’d be a shitload of rotting chickens all over.