Everybody eats meat, which means we buy meat, and if we all went vegan, the economy might take a major downfall.
This is the real world, we've been hunting and killing cows and stuff to survive for god knows how long
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What kind of downfall are you talking about? Bear in mind that it wouldn't be an instantaneous shift. If the world were to become vegan, it would happen gradually, and the economy would have time to move with it. Collapses usually require sudden, unexpected changes.