Everyone becomes a vegetarian ,You think farmers are going to keep raising them farm animals for nothing.If you set them all free that hurts the natural wild life in the area.So what would happen if everyone became vegetarians it would be the end of a lot of animals lives and maybe as a species.There used to be cows but no more,chickens I remember chickens but that was a long time ago.
Their more about emotions with no thought to the results ,Stop all hunting ,the populations of deers or whatever out strip their habitats ,they then starve in large numbers or a disease wipes them out.
That's nature, that's how it works.
I'm not about all out slaughter but a reasonable hunting program ,a conservation type approach is what does more to keep animal populations healthy and alive.
I believe most members of Peta live in the city,have no clue about nature or how it works and want to make it like animals live like people.They don't