People are conflating multiple issues here.
Pure veganism is about the animals, not the environment or health.
They are perceived benefits but this is not the primary motivator for most vegans and so arguing for/against veganism on the basis of environmental or health grounds is like arguing for/against meat consumption with similar reasoning. Everyone will find the scientific study that backs their lifestyle. Anecdotes are just that.
On ethical grounds however, I don't think we can ever justify the sheer number of animals slaughtered and the suffering caused through factory farming. They are sentient, they feel pain, they do not want to die. We have become greedy - nobody needs as much meat as we're consuming; meat eating is a privilege.
A PP said they only eat red meat a little, twice a week. Imagine saying that to somebody 100 years ago? Twice a week is actually quite a lot in my opinion.
I do think you can justify people hunting for food and making the best possible use of the animal in locations where plant based alternatives are not a practicality.
I also think there are people who are limited by illness and may need animal products to live healthily but also think they should strive to buy the highest welfare meat possible.
It all depends on your own moral compass and situation. I am lucky to be healthy, knowledgeable about cooking and in a society where plant based foods are freely accessed. I also consider myself an animal lover and so choose not to eat them but you can't make people change their ethics.