Veganism is a more ethical diet than meat eating and dairy eating, if you consider the lives and suffering of nonhuman animals to matter in your frame of ethics. It's not "pure" because it's impossible to have a completely pure diet that causes no harm to other beings (especially under capitalism), but veganism strives to minimise harm to the extent that it is possible and practicable. I'm certainly not "pure". The fact you can't ever be pure though doesn't mean trying to be better is not a worthy goal.

The fact that you need to supplement b12 as a vegan is not enough to dismiss it as an ethical choice for animal welfare and for the planet imo. Especially given that many people need to supplement on an omnivore diet anyway (as I did before I was vegan, as my non vegan friend with celiac disease has to supplement iron because she can't absorb it well). Vegans on average tend to be healthier than the average omnivore because they have to be mindful about nutrition and also eat more vegetables than most. But obviously not everyone is like that in a movement of millions of people. So you can find fringe people in any movement. Would you like people to look at the "Liver King" or Jordan Peterson and say that represents all or most meat eaters?