You wrote, "One can even paint a fairly idyllic bioconservative world where human enhancement is impossible and people don't interact with advanced technology any more, they live in some kind of rural or hunter-gatherer world where the majority of suffering and disease (apart from death, perhaps) is eliminated by a superintelligent singleton."

If it would make sense for that singleton to look after humans that way, it would make sense for humans to look after dogs, cats, and ants that way. And we don't.