His rebuttal would be that he plans to genocide obligate carnivores in a way which causes them neither suffering nor premature death. It would involve neutering as it is performed regularly today.
My counter-rebuttal would be that his final solution still causes massive death, which is harmful even if there is no pain. The vegan would have to kill countless mammals, birds, reptiles, and insects to avoid suffering. This would ironically cause untold suffering in nature. Other creatures would overpopulate the planet, leading to mass starvation and extinction of even more animals as food runs out.
This doesn't even touch on omnivores. Because even these animals are "obligate carnivores". Bears, horses, and gorillas will not eat meat, not because they're making a moral commitment but because the opportunity to eat meat does not exist. When the chance presents itself, these animals eat meat as much as any other "obligate carnivore".