Those animals only exist because we farm them, so farming them is better for them than not
It is true that fewer animals would be bred, but an animal that is never born loses nothing. Bringing a being into existence does not give you the right to kill it later. If it did, the same reasoning would excuse almost any treatment of anything we chose to breed.
The animals on a well-run farm exist only because people want to eat them. Without that demand they would never be born at all. If their lives are genuinely good, with space, grazing and care, then a good life ending in one bad day seems better than no life. If the animals could choose, they might well choose to exist on those terms.
Causing a being to exist does not give you a claim on its life. Parents bring children into existence, and nobody thinks that entitles them to end those lives once they become inconvenient or useful. The fact of having bred an animal is the reason it exists. It is not a reason it may be killed.
The other half of the trade is empty. An animal that is never bred is not deprived of anything. There is no one waiting in the wings to be disappointed. The animal that does exist can be harmed, and slaughter harms it. So the comparison is not between a good life and a lost one. It is between a life cut short and no one at all.
The argument also proves far too much. A broiler chicken in an intensive shed would not exist without that shed either. If existence outweighs what is done afterwards, the reasoning excuses every system, not only the good ones. It would even suggest we ought to breed as many animals as possible. Few people accept that duty, which shows existence is not really doing the work here.
Welfare differences between farms are real. A well-run regenerative farm can give an animal a far better life than an intensive one, and that matters. It changes how much harm is done. It does not change whether the killing was necessary. A beef animal that could live twenty years is usually slaughtered before three. Separation, transport and the slaughterhouse are still part of the life on offer.
The imagined choice offered to the animal has only two options: a life ending in slaughter, or nothing. A third option, a life that is not ended early, is missing only because it does not pay. That is a fact about the farmer's economics, not a favour to the animal.