Okay, I'm going to ignore all the drama and tetchiness in this thread and just answer the original question as best I can (but first, I agree with Ronnie in his suggestion that you read the China Study. Very informative and actually what inspired me to study what I'm studying at university).
Yeah, maybe the idea of a vegan world seems impossible to you, Cassie. I can see how it would. It's unlikely we'll ever achieve that, but not impossible. Remember that not so long ago the same was thought about a woman working and voting, or a person from any origin that wasn't anglo-saxon having equal rights to Caucasians. Look at us now. We progressed, and the impossible is now reality. It's a beautiful thing, right?
Should this hypothetical vegan world eventuate, it will not happen overnight. It probably won't even happen over five years. It will probably be the result of a very gradual decrease in the consumption of animal products (food or otherwise. Fur coats are still consumption), and probably be the end result of a many decades long shift in human lifestyle. Thus, these domesticated food animals (cows, pigs, sheep, chickens, etc. as we know them) will gradually decrease in number. Supply always meets demand, so as demand gradually decreases, so will supply. Land used to grow feed for animals will now feed up to 20 times the number of people.
[[I should point out that the two largest feed-the-hungry programs in the world are vegetarian. Not for ethics, but because it feeds many, many more people.]]
The same gradual process applies to the nonhuman animal population regulating itself. Any harm done to the animal population and their various corresponding ecosystems was inflicted by humans, and while I feel we ought to continue working to save those we have endangered but not yet lost, I feel that we can gradually phase out human interference over a period of time. Ecosystems are not dependent on human influence.
Finally, I do not try to push my lifestyle on others, I do not try to force veganism on others. In fact, more often than not, it's meat eaters who try and inflict their diets and lifestyles on me. I am in a long term, committed, happy relationship with a wonderful man who I have no accepted will probably always eat meat. But my lifestyle is one he respects, and through talking to me he has even improved his health. He eats less McDonalds and more salad, and that is enough to make me happy.
Rather than be pushy or demanding, I find that I cook vegan food for my friends, I answer any questions they have, and discuss nutrition with them. Through example I have encouraged people to cut down on meat or go vegetarian. I have never convinced someone by trying to force it on them.
I know this is a long post, but if you took the time to read it all, thank you. I hope I answered your question. Finally, I'd like to reiterate that while this is a very an unlikely scenario, it's not impossible, and even if we never achieve 100% what we want, the ultimate goal is simply positive change, and a lessening of cruelty.