You are appealing to emotion.
Elite athletes are already outliers within the human race however within athletics there are extreme outliers like Bolt and Phelps who have astronomically rare genetic advantages. To be representative they need to be discounted. Not even the Olympics is N number of Usain Bolts racing against each other. It is N number + Usain Bolt.
Donnie McFayden may have trained as hard as he thought he could. That does not mean he was in the optimum training environment, he apportioned his time correctly or he trained with focus required to be better than he is. His training habits may have become effective simply too late in his career (Maybe he took training seriously at 8 years old but his competitors (in team and other team) took it seriously at 4. Maybe he is absolutely outstanding but the team failed him.
The factors are numerous. It might just be he is unlucky; his coaches didn't recognise his brilliance or they played him out of position. The point is not why is Donnie not the absolute best in history. It is 'Can anyone with the basic requirements reach the level of playing international rugby with the right focus and training and luck - the answer is yes, yes they can.'
They literally just have to want it enough and have the resources (time, money, family, coaches) in place to achieve it.
Anyone can be a Chess Grandmaster with enough study. Proven by Laszlo who specifically taught his children chess in order to prove geniuses are made/trained, not born."
http://www.psychologytoday.com/articles/200506/the-grandmast...
http://en.wikipedia.org/wiki/L%C3%A1szl%C3%B3_Polg%C3%A1r
Also proven by the Williams sisters whose father decided* they would be tennis champions and worked them mercilessly towards that goal using any equipment to hand. Unless your hypothesis is that -
Two genius tennis prodigies were born with the gifts required to be a tennis prodigy and their father just happened to settle on the sport of Tennis? I think those odds are considerably more unlikely than "anyone can be a Tennis champion with the basic requirements and dedication which is what the Williams sisters had."