It's not that simple. If the world consumes a million less pigs every year, that's not a million pigs that get to live happy, wild pig lives; it's a million pigs that will never exist, because we don't need to make them.

You could certainly make the argument that non-existence is preferable to the life of a present-day factory-farmed animal. I wouldn't contest that. But perhaps there is some value of ethical treatment for which "a few years of peaceful farm life followed by a painless death" is preferable to non-existence.