All the great philosophers knew that a vegan or vegetarian diet is better for our health, emotional wellbeing, mind and spirit. Why don't we all follow the greatest thinkers of history and stop killing and eating living creatures? There is more than enough plant food for all the people on earth.
Most of the suffering on earth will disappear immediately if everybody starts to eat plant food. Everybody will have enough food and animals are no longer badly treated. A person who can kill an animal, can kill another human being. A vegetarian or vegan person simply can't. All war would be over!
What is your definition of a "great" philosopher? And what evidence is there that these Greats were vegetarians in the modern sense? Plato and Kant were arguably great philosophers, but it's highly unlikely that either was a vegetarian.
I also take issue with your characterization of a completely vegetarian planet (as well as your statement that a vegetarian or vegan person is incapable of killing). You are making categorical statements without evidence to back them up.
I have no problem with vegetarianism, being a vegetarian myself. What I have a problem with is faulty logic and propaganda.