I would say to them that the topic is irrelevant. You can't really make historical or evolutionary arguments about diets. Our modern diets are far removed from what our ancestors ate due to agriculture. Even the typical omnivore diet is dependent on domesticated animals and new varieties of fruits, vegetables, grains and legumes that haven't existed for very long on an evolutionary time scale. And, as you implied, we don't have to base our every decision on survival these days. We can weigh options and do what we think is right.
Today, we know that a well-planned vegan diet can be just as health, if not healthier, than an omnivore diet. B12 supplementation is vital, as is eating a variety of foods. Even the American Dietetics Association and the USDA state that a well-planned vegan diet is healthy and appropriate for all stages in life (including pregnancy and early childhood).