Imagine for a second if millions of years of evolution were not wasted poorly, and the human brain is actually a Pareto-frontier of efficiency-generality of intelligence. And even in principle, we can only approach it with AI, never surpass it. If this is the case AI would never be able to truly replace humans and it would be always more financially meaningful to build specialist AIs that enhance humans, rather than trying to replace them completely. One good ending that is not that improbable.