So, in all of human history, people who ate only plants were frail and prone to sickness and suffering? The ONLY dietary argument you can make against veganism currently is that of Vitamin B12, which people can get anyway from B12-fortified pills/food, or even top soil crops, depending on what country they're from. For that one argument, you have a vast multitude that can be used against you. And all that isn't even factoring in damage to the environment/animals.