If an AI is created that has consciousness and is smarter and more moral than humans, it could lead our world better than we currently do. It could genetically modify us to make humankind a more ideal species. These AIs could be better humans than humans can possibly be.
In 100-200 years, "kill all humans" could actually be the morally-best choice. By comparison, we could be anachronistic barbarians suitable for nothing but the novelty of viewing in a zoo.
There are certainly risks from passing the baton to a different unknown future lifeform, but I don't think we can have a priori certainty that replacing humans with AI will be inherently bad.