What about grazing cows in arid lands that can’t be used to grow crops? Isn’t that sustainable?
In some cases it’s true that you can raise animals in conditions in which you couldn’t raise other food sources. But the goal is not to use every square foot of land on the planet to produce food for humans. Many studies have shown that we can grow more than enough plant-based protein to sustain everyone on earth, while also freeing up land that was formerly used to grow corn and soy to feed to animals on factory farms, allowing that land to be re-forested or returned to meadow or marshland. Returning land to its native ecosystems, populated by native animals rather than invasive domesticated animals, is the most sustainable option.
Much has been made of Allan Savory's work and TED Talk promoting cattle grazing as a solution to climate change. His work has been scientifically disproven, and there are several articles debunking his theories: