One important note I might add is that meat contains some nutrients that can be difficult to get elsewhere especially for vegetarians. Meat has a lot of iron, zinc, and magnesium. Healthy fats too.
My opinion is that a lot of vegans can get into trouble if they don't take steps to make sure they find these nutrients elsewhere in their diet. Vegetarianism can lead to copper toxicity for this reason. That certainly doesn't mean it can't work for ppl that replace these nutrients with other sources.