Food proteins aren't the same. You have to eat a lot more grams of plant protein to get the same amount of essential amino acids as if you eat beef, for example (I think the ratio is 2,5 to 1 on average, from memory). A side effect of this means that you're eating a higher volume of included carbohydrate to get the same effect, which can result in weight gain.
Some ppl can tolerate the raised level and so the diet is good for them, some cannot and it's not good for them. I wasn't writing to state that the diet was bad - for many it's good, but then for many it's not. I wouldn't ever discourage anyone for trying something new, but have to accept we are all different and it's not one size fits all.
For example, I can eat steak and cheese and drink milk with relatively zero bad side effects, but if I have a small bowl of pasta or some bread, Im ****ting through the eye of a needle an hour later and feel hungover the next day, so eaten gluten free since my 40s and feel better for it, but then most can eat wheat with no problems at all, so my diet isn't good for everyone, but for some, it can be.
But regarding the ethical reasons for veganism, you're probably in the right and our great grandchildren will see how we have treated sentient beings in farming the same way that we regard slavery. But clearly, I don't think about this enough to change.