Plato used his allegory of the cave to demonstrate man’s attachment to the ideal forms. After spending his life in a cave, coming to the surface and seeing the sun was a new kind of experience.
In Buddhism, it is said that desire is at the root of human suffering. What if the desire for perfection–i.e., for life to take on the shape of Plato’s forms–is an example of this?
Perhaps proving the Buddha’s noble truth #4, that we can overcome suffering, consists in shedding our fantasy of becoming ideal humans. Assuming this, transhumanism and posthumanism may go too far!