> Meanwhile, of course a vegan diet is better for the environment
This is not a matter of course. Humans are part of the environment, and it is not a matter of course that a vegan diet is healthy for humans. The idea (I don't want to say fact, but I believe it) that meat and dairy consumption/industry are currently happening at a scale that is harmful does not mean that meat and dairy are bad for the environment.
I don't understand the structure of your argument. What do you mean by "not a matter of course"? There is plenty of evidence that 1) vegan diets are associated with improved health outcomes, broadly speaking and 2) animal consumption has a disproportionate environmental impact for the nutrients obtained (which is unsurprising from a physics perspective, since the animals eat plants that humans could eat directly).
Perhaps these things are not "inherently" true in that one can easily come up with edge-case counterexamples, but they are true on the whole.
You say "of course" when something is a matter of course. It is similar to begging the question.
1. Compared to what?
2. Animal consumption or current patterns of animal consumption?
Note too that I do not claim that meat and dairy are proven more healthy, or better at any scale. Just that we must be careful not to let greenwashing make us think they're evil and gross as a kneejerk reaction.
This is not a matter of course. Humans are part of the environment, and it is not a matter of course that a vegan diet is healthy for humans. The idea (I don't want to say fact, but I believe it) that meat and dairy consumption/industry are currently happening at a scale that is harmful does not mean that meat and dairy are bad for the environment.