A sketch of the argument: We have a duty to prevent suffering when it is possible. But unbelievable suffering happens whenever we allow nature to take its usual course. Soon, however, we will not require nature to take its course at all. We will be able to live without it. Engineering friendly animals, as recommended by Charles Fourier, would take centuries longer than extermination, and during that time, animals would continue to suffer. The right thing to do then will be to dismantle the entire suffering, misery-ridden ecosystem and replace it with machines that maintain livable conditions for humans, while neither inflicting misery nor experiencing it.
(Note: I don't actually endorse this argument. I raise it because I think it challenges some widely shared beliefs.)
