While the environmental benefits of being vegan are tremendous, and probably the single best thing one can do for the environment, the environmental benefits do not compare in importance to the moral reasons for going vegan.
Sentient nonhuman beings are just like us in all of the morally relevant ways. They value their lives. They strive as hard as they can to live. They feel pain intensely. It is wrong to breed, exploit, torture, and kill them for food or any other preferences.
Focusing on the environmental benefits of being vegan is therefore much like focusing on the environmental benefits of avoiding an unjust war. True, war and animal agriculture are both “tough on the environment” (to say the least), but to oppose an unjust war and the killing of innocent civilians, or animal agriculture, by appealing to environmental protection ironically misses the point in the same way.
We need not be silent about the environmental benefits of veganism, but when we do address such benefits, we should imply or point out that, while great, they are very much incidental to the grave moral wrong of exploiting and unnecessarily breeding and killing the innocent.