What about "He doesn't have a net advantage" is hard to understand?
But that seems like a weird rule to me. Suppose we could demonstrate that his artificial legs quintupled his speed relative to the average Olympic runner. Okay that would clearly be an advantage, so that would be bad...so how would he react? If he were rational, he'd buy less effective prosthetics, but unless he is abnormally scrupulous he would not want to buy ones that made him just as fast as the average Olympic runner (in which case he'd lose the race, as one of the competitors is bound to be above average), he'd want ones that put him in the top 10%, or 5%, or 1% (or in the extreme that made him just 1% faster than the fastest reported runner).
There is no such thing as an "unfair" advantage, except subjectively, and even what is an "advantage" has some subjectivity to it in certain circumstances...but athletic competitions tend not to be terribly subjective. Winners and losers are determined by largely objective criteria. Or at least that's the way it used to be.
If we want to allow in mechanical aids so long as they are "fair" then we undermine what I see as essence of the sport--the objective determination of a victor, because any win by the augmented athlete will forever be open to question. We may hide the subjectivity we've just added in a pre-race determination, rather than in the final criteria of who crosses the finish line first, but that doesn't make the guy's wins any less open to valid disagreements.
As for Tiassa's question of who would give up their legs for prosthetics, very few would, at least right now. Give the prosthetics a decade or more to advance, and I think the numbers will start to increase dramatically, especially if you have dedicated your life to a sport, and the prosthetics increasingly start to seem like a de facto necessity to remain in the top tier of all athletes.
I can as easily ask who would want to risk getting cancer to be good at college football? The answer today is that a lot of kids can and do volunteer to take that risk for that purpose. It my have been different before steroid use started to become prevalent, but then you reach a tipping point and those within a given community (like the community of top athletes) do crazy things.