We still need a few breakthroughs for prostheses to reach a level that they can compete with the original body part. For artificial limbs brain-machine interfaces are needed for better control, as well as advances in engineering for better mobility. Artificial organs can be grown from the patients own cells and attached to a designed graft, opening the possibility of enhancing the functionality.
In some fields prostheses can already outclass the original part, as demonstrated by the double amputee Hugh Herr, who actually improved his climbing capabilities by modifying his prosthetic legs.
When will a person voluntarily replaces a fully functioning body part with a prosthetic one, or will people decide not to?