In 2000 Kenya had 8 men under 13 minutes. In 2010, they also had 8 men under 13 minutes. In the last THREE seasons (2016/17/18) they had just two men barely under 13 minutes (both 12:59).
Yes and conincidentally is in the last three years that there are "more" control in Kenya about doping, the main factor
is of course that the money are in the road races so many talent just go direct to the marathon.
About Kipchoge dominance , well he 's out of competition, is not even possible compare him to others Kenya runners for many reasons. Many they just CANT or dont want to take to much risk.
Now we have seen London Marathon with Brigid Kosgei in 2:18. I' m sure that people like Mary Keitany (very wise) dont want to spoil
her great career just now risking a lot with peds. And the same Vivian Cheruiyot. These days is not like 20 years ago you dont have to pass a certain limit, is a delicate balance, If you pass it, yes you can have an incredible performance and beat the world record but then
the risk is that you end up like Abraham Kiptum.