B12 is not an “extensive amount of supplements”. Furthermore your average contemporary nonvegan likely consumes way more supplements than a vegan just indirectly through livestock feed, including B12. Even ignoring the “animal” sources though, people these days consume a lot of supplements from cereals, energy drinks, table salt, and more. I’m not sure why relying on supplements would even be a gotcha in the first place though.

Furthermore I’m not sure why it would matter that there exist healthy diets that include meat, like you said a mostly plant foods diet but including some fish. If anything that reinforces that there are other reasons for people abstaining entirely from animal foods. Veganism is way more than a diet, it is an ethical stance, and extends to far more than what we eat. It just so happens that what we eat leads to some of the most egregious abuses and also butts heads with culture because what we eat and what others eat is so important to us as humans. That’s why the way vegans choose to eat gets so much attention.