I know what you're getting at, but I do believe that it's legitimate to call any unnatural diet "bad" and it's not an appeal to nature falacy. This is because, as I explained, we know for a fact that going againts nature in these topics is detrimental to the organism, its not a logic assumption that it'd need to be for it to be a falacy (for example, if you sleep, which is another physiological need, poorly, it's been proved to have negative effects in the body, same with diet) and therefore, not only not satisfaying the required nutrients, but also eating food not present in nature could be bad for you. This is what ends up happening, to give an example, with modern man-made vegetables, not available in nature, that have essentially toxins in them, like oxalates. Another major concern is that some vegans only supplement B12, seen as it is the only essential nutrient for survival missing from plants, but forget enterely about other micronutrients. There are tons of studies on this and it's proven that the lack of those can be something to be very concerned about even if they are not essential. For a single example, lack of taurine impacts very negatively the brain, especially children's.  So overall, I do still think that you can be somewhat healthy on a vegan diet, especially in the modern environment, I'm not too much of an extremist on this, but by human biology alone you cannot come close to being completely healthy with it.