It's the doping that is killing the marathon, along with pretty much every other event in the sport of track and field.
How does doping kill the sport? It becomes the final differentiating factor among the very best as to who will edge out the other.
So, it's not so much the specialness of the individual, but rather the effect of the drug.
The rash of Kenyans running so fast over the marathon is testimony to the new mode of operation in Kenya, where doping has become the get-rich-quick card that many athletes, coaches, and agents are playing.
Obviously, it's been going on in Russia for decades. The US sprinters are notorious dopers.
Where there is money, there tends to be corruption. There's money in the marathon, now the sport has become infested with druggies.