Sterilizing them & keeping them on repurposed farms with basically decent standards of living until they die of natural causes is probably the most humane route. However, in this hypothetical scenario we would have to contend with the constraint problem of necessary land/food/water supporting these animals with their (often quite long) lifespans. In reality it will probably be a patchwork of euthanasia with isolated efforts to save as many as possible. The unfortunate aspect of creating enormous death machines is that even when you shut them off they take time to grind to a halt. At least we will not continue to bring billions of animal lives into existence for the purpose of suffering & slaughter.