What would happen to all the animals?
Demand falls gradually and breeding falls with it. Nobody is proposing to open the gates on eighty billion animals at once.
Billions of cows, pigs, chickens and sheep exist right now only because farming keeps them, and they cannot simply be released or left to fend for themselves. If people stopped buying animal products, farmers would have no reason to feed and house these animals, and nobody else has the land, money or willingness to care for them, so the likely outcome would be mass slaughter or neglect on a scale worse than anything farming does today. Before asking everyone to stop, vegans owe an account of what actually happens to the animals already here.
Farmed animal populations are continuously replenished by deliberate breeding, calibrated to expected demand. They are not a standing population that would need somewhere to go; they are a flow that slows when the demand behind it slows.
In a gradual transition (the only kind on offer) producers breed fewer animals each cycle. Existing animals live out whatever their intended lifespan would have been, some in sanctuaries, most as now. The scenario where a vast surplus suddenly requires disposal is not a stage any realistic path passes through, and imagining it as the alternative is what makes the objection feel forceful.
Replacing someone's actual position with a weaker, distorted version, then refuting that instead. The original argument is left untouched.
How it applies here: Assumes an overnight switch with existing herds released en masse, which is neither the proposal nor how consumption changes.
Presenting two options as though they were the only ones available when further options exist. Also called a false dichotomy, or black-and-white thinking.
How it applies here: Presents the options as the present system or sudden collapse, leaving out the gradual reduction that is what anyone actually advocates.