Indoor plumbing is not natural, so why do we have it? Our brains allow it. Any reduction in meat consumption helps. Although bonobos and chimps eat 97% plants and insects, most have less than 3% meat consumption. War among them is even more rare.
Although violence is natural, it too is not as likely as cooperation. our brains should allow us to end it, just as with outdoor defecation.
Best argument for veggies: nothing unsustainable lasts forever.