First, nobody said anything about eating only meat as a diet. I'd wager zero people do that - you would quickly develop vitamin deficiencies and ultimately fall ill and die.
Secondly, you've moved the goal posts to vegetarianism, which is not veganism.
Third, just because a vegetarian diet can be sustainable (in the sense you won't fall ill) doesn't mean it's the natural diet for a human. It's not... Your teeth and many other parts of your human body tell us differently.
Fourth, there are several cultures on this planet that strongly favor vegetarianism, but none are strictly vegan and few are strictly vegetarian. As already pointed out, vegans often have to take supplements to remain healthy. That's biology telling you that diet isn't natural.
Lastly, deciding to be a vegan because you believe current agriculture practices are harmful to the environment is a choice based off your beliefs and therefore is an ideology. That was the original assertion.
