Where will all the animals go? We’ll be overrun by cows/chickens/pigs.
We currently kill nearly 60 billion land animals a year, and between 1 and 3 trillion aquatic animals for food and clothing.
In the case of aquatic animals, we have killed so many that 95% of fish stocks have been depleted, and many species are on the verge of extinction. We would do well to stop fishing altogether and allow our oceans to be ‘overrun’.
As for the land animals, we specifically breed them in huge numbers. Some species have had their phenotype so drastically altered for optimum meat production that they can no longer breed normally, and have to be artificially inseminated en masse.
In reality, we are not all going to go vegan overnight. As more of us do though, demand will fall, and supply will dwindle accordingly. We will stop breeding so many animals. Eventually, the number left will be small enough to handle their emancipation with relative ease.
If we want to, we can keep a population of each type of farmed animal on reserves to prevent their extinction, though personally I would be content to allow domesticated species to die out.
If we allow farmed animals, whose breeding habits are currently strictly regulated, to breed as they choose, they will begin to resemble their historical phenotypes within a few generations. Soon they will no longer be reliant on human intervention for survival. There will be more land available for them to live on, as a vegan diet requires about 20 times less land to produce the same amount of food as an omnivorous diet. They will not pose a threat to wildlife and the environment anywhere near as much as animal agriculture does. We presently use 26% of arable land to graze farm animals, and a further 33% to grow crops to feed to farm animals. Farming animals is far more hazardous to the planet than releasing them.
In short, we will not be overrun by chickens.
Even if we all turn vegan today, I think it would be the least we could do to turn farms into reserves, allow all the farmed animals alive today to live out their natural lives in peace, and stop breeding new ones. We found the time and resources to accommodate them when we ate them, we can do the same out of compassion. Ending animal agriculture will mean there are more resources and space available for growing a more diverse range of crops, and will abrogate the need for decimating rainforest and woodland to make space for cattle.
The logistics of dealing with the problem of what to do with animals that we brought into existence in order to kill them for our pleasure and convenience is not a justification for continuing to breed, intern and slaughter them.