Here are my thoughts on the topic: The brain has allowed humanity to transcend biological evolution and replace it with technological evolution. Man has created tools to make life easier in almost every regard. So it seems almost inevitable that we look no further than ourselves to develop technology around. I love my biological appendages as much as the next guy who up until now never thought about any alternatives. However, what happens when science gets us to the point where the prosthetics arms and legs are superior in every way to our biological ones? At that point (assuming social stigma fades away), being a cyborg will not only be better than organic, but actually sought after. Imagine having the look and feel of a model without having to put in the time to perfectly calculate a diet. Imagine having superhuman strength without stepping foot in a gym once. Maintenance? Maybe a yearly tune up. At this point, humanity would have nothing but our brain left that has any semblance of "old-world" humanity. What happens when we crack the neurological code and can now augment our ability to remember, to think to perceive? When this technology comes to a point where there's no downside and severely upsides the change over will be inevitable.
I think this is exactly right - we have largely removed ourselves from the normal evolutionary process of natural selection. I myself would very likely have died at age 10 from appendicitis; there are myriad other examples. The trend towards cybernetic enhancement goes back at least as far as eyeglasses, and is not likely to slow down any time soon, unless catastrophe knocks us all back into the natural selection game.
So aren't we assuming being non-biological is better? Just based on our own intellectual missteps?