I think if you want to be vegan that's up to you, but having thought about and discussed the issue at length with a fairly large number of people (there's a reason why it comes up but it's not necessary for me to go into it) I find the arguments for it particularly poor.
The poster who mentioned that various species would pretty much go extinct if everyone turned vegan is bang on, absolutely correct. The sheep and cows etc that vegans enjoy seeing just wouldn't be there if we didn't want something from them.
What about animals that die of natural causes (because animals do die, yes?) - can we eat those? If not, why not? I'm fully on board with the ethical treatment of animals, farmed or otherwise and regs in the UK are pretty darn tight.
We also mustn't overlook the fact that we live in the monumentally privileged West, where a vegan diet is not only possible but fairly readily available. If we were living as little as a few hundred years ago, or in many parts of the world even today we'd have little choice in the matter, meat would definitely be on the proverbial menu.
I wonder how many vegans have ever walked behind a plough or a combine and seen the utter carnage they cause. Millions of worms die every year so vegans can have their pea and sweetcorn smoothies.
I also find veganism to be idealistic. Just because you walk right on by the big juicy ribeye in the supermarket doesn't change a thing. The person behind you is going to buy it and eat it and that's the way it'll likely always be. The only way for veganism to really hit hard in the meat/dairy industries is if it gains massive traction and that doesn't seem likely for all manner of reasons (people want the products, a lot of people just don't give a shit about the subject etc)
Lions are pretty rare nowadays. If everyone heads to McDonalds and starts demanding a big old McSimba burger you watch the lion population explode. Might be a sad way to think about it, but it's still true.