I'll go vegan when every carnivorous animal species on the planet also goes vegan.

You can get all the nutrients you need without eating animal products but you will struggle to get Vitamin B12 unless you eat foods with supplements. Hmm. If we didn't keep animals for food or dairy, much land unsuitable for crops would go out of use, meaning less food, though I suppose we could always plant trees. Think upland pastures or the areas which require seasonal animal movements because of drought, for example. And developing-world farmers can't always afford artificial fertilisers, so many depend on animal manure. Having said all that, it behoves all of us to eat a bit less meat and to always question hard the welfare of the animals that provide our meat. I do find it galling to read that around half the wheat and barley production of the UK goes for animal feed. That seems entirely wrong to me. But let's keep veganism/vegetarianism as personal choices, free from militant crusading. There are no black-and-whites in this.