A growing amount of evidence suggests that the healthiest diet you can have is a whole-food plant-based diet. I have been moving more towards this over the last 12 months or so. I cut out dairy a year ago, started eating less meat and more plant based foods. I've now cut out meat & eggs altogether, so I'm eating what could be described as a vegan diet. I've honestly never felt better and I'm certainly eating very well. People don't need meat, they choose to eat it. I don't have an issue with people eating what they want to eat. I still cook meat for my kids and my partner, I've just decided it's not for me anymore. It's possible to have a really good plant based diet if you learn about nutrition and know how to cook.
There are lots of vegans and vegetarians that eat poor diets, the same as there are lots of omnivores who eat poor diets. I think nutrition should be taught in schools.