Ah, so your point was that we take B12 supplements for the nutrition, not just for taste pleasure. I genuinely had not understood that.
I am aware of B12 being recommended to supplement. Personally, I don’t worry much about it, because my oat milk is fortified, my vegan cheese is fortified, even the multi-vitamin juice in my fridge has B12 in it. And the supplements are dirt-cheap, too. But yeah, sure, people in different regions might not have it as easy in this regard.
The thing is, though, if we disregard those people, and also disregard all the meat-eaters who genuinely care about their nutrition and genuinely believe that they can only get it from meat, i.e. we let those eat their meat,
then that still leaves a huge number of people, who would significantly improve their diet, if they significantly reduced their meat intake (or cut it out and replaced it with appropriate vegetables + supplements/fortified stuff).
Nutritional experts have been screaming for decades that people should eat their veggies. Because those contain a massive range of vitamins, which the average person is not getting enough of. And if you’re eating enough veggies, then you need to cut back meat intake far below the average or do a lot of sport, otherwise you’re just consuming too much food.
Ultimately, why the nutrition argument is rarely taken serious, is because the average meat-eater is so far removed from eating healthy that they probably don’t even know what B12 is.
I guess, if you want the sensitive version of the strawman argument which you just came up with, that apparently the hivemind of vegans says that omnivores eat meat only for taste pleasure, then as a certified Vegan™ and part of the hivemind, I am glad to tell you: Not all omnivores eat meat only for taste pleasure. But a significant portion of those living in developed countries could easily go vegan without sacrificing nutritional quality and rather even improving it.