It's important to acknowledge that, and also that the question of sustainability arises only because of population growth. Growing demand encroaches on land (for a variety of reasons) at either a higher or lower rate, but still encroaches. A stagnant global growth rate would render the problem moot, which is going to happen in 100 or so years. In the meantime we're dealing with environmental problems that require immediate rectifying, and varying consumer habits would be wholly insufficient to do that, though still welcome. Waste in particular could be disrupted with better policy, and frankly, so could agriculture. Meat is subsidized. It's practically a matter of policy to encourage consumption as it stands. To say nothing of regulation, the potential for less land-intensive feed, etc.