Veganism and to a lesser extent vegetarianism are the brainchildren of modern civilization (and a considerably bourgeois segment of it) that is supported by unsustainable supply chains and cities. Most likely much of veganism is another abstraction of bourgeois elitism.
Obviously meat isn’t the cause of our sustainability deficiency. Our most sustainable vegetable crops are probably less sustainable than the Native American’s buffalo hunts that predated them by hundreds of years.
The assertion that vegan and vegetarian diets are more ethical than omnivore diets is also dicey and probably also stems from elitism. Animals eat each other, often in painful and terrible ways. I hear the argument that, because humans are rational and can make decisions, we're "above" animals and therefore can choose to abstain from eating meat. Our rationality also brings us VX gas so certainly rationality alone does not imply moral authority. We humans often make decisions with the information and understanding at our disposal only to find those decisions have unintended consequences. Even in the very bodies that we consider to be under our individual purview there rage endless battles of consumption in microorganisms, the cycles of life and death that constitute life as we know it. To make the claim that meat consumption is unethical is to take ethical exception with nature itself. That is a difficult position to hold, especially given most of the vegans and vegetarians hold the abstract concept of nature (as opposed to their very existence as a natural process of consumption) in high esteem.