I think what it comes down to is that the body was built in a certain manner. Any replacement won't function the same way the original does, and therefore adaptations must happen and until you rebuild the whole body, the replacement can never be better than the original design, because it wasn't designed in that manner in the first place.
As for the argument of shoes. Because of shoes, we have aplastic deformation of the feet. Because of shoes, we are at a loss of natural functional and structural stability. Not as good as the original. But necessary, yes.
As for eye correction, we have eye correction because kids sit in rooms staring at a board 20 feet away all day, we don't need the correction, not if we didn't screw with the natural eye in the first place by underdeveloping the eye muscles.
All modern medicine has side effects. All modern medicine is a stress to both kidney and liver at LEAST. All modern medicine is a stress to the body, and the body fails when there is too much modern medicine in it. Because modern medicine doesn't take into account the... once again, original design.
There is a law that simply states that structure and function are linked. If you use something in a way that the body wasn't meant to be used, then function suffers, long term or short term. So far, modern technology has failed in every manner to mimic exactly what the body was designed to do. That is why they are going into stem cell research, because they want to use what is already imbedded, already designed, in order to create something natural.
