Regardless of how much good vegan diets do for the climate (which is already quite a lot), it is important to note that most vegans, myself included, are vegan for ethical reasons.
The vast majority of humans can thrive on a diet the does not include animal-based ingredients. Even B12 is easier to get, as the only reason meat is currently a good source of B12 is due to the animals being fed supplements. You are still getting supplemented B12, just one step removed.
There are some people who cannot live without meat, which is why we also advocate for lab-grown meat. But in my own experience, the majority of people who say they need to eat meat for health reasons do so more for convenience or taste reasons. They often only mention these health issues after they are confronted with the reality of animal agriculture, and will continue to claim illness even when they have never tried alternatives.
At the end of the day, ending the life of a sentient being simply because you want 15 minutes of pleasure in the form of a particular kind of food is morally abhorrent. If it is considered abuse to hit an animal for not doing what you want (which is the case for every animal except those in factory farms), then killing them for an even less consequential reason should also be seen as abuse.
All of the above is not even to mention the absolute horrors that workers endure in slaughterhouses, the number of zoonotic diseases people die from as a result of squalid conditions and overuse of antibiotics in factory farms, and the fact that we grow more food to feed livestock than we do to feed humans, despite meat and dairy making up less than half our food supply.
Animal agriculture is unnecessary at best, and downright psychopathic at worst. Animals are not commodities; they are living, breathing, feeling beings that have been proven time and again to experience suffering and pain. Going vegan will not hurt you, the climate, your health, your children, or anyone else, and it will ensure that another creature is not forced to live a short, torturous, and painful existence simply because you want a particular kind of sandwich.