Kenyan diet has nothing to do with their running success
I wouldn't use long distance athletes as a means to prove the disadvantages of a paleo diet. It is well known that such athletes do not live long and routinely suffer heart attacks. Not so much because of their diets, but because they push their bodies so hard. However there are tribes such as the tarahumara who are well known to run 300-400 km daily over several days, and living into their 80s.
However there are also native peoples in the arctic who live well into their 80s as well, eating diets consisting of mostly meat. As well as people who live in siberia who live well past 80, again eating diets consisting mostly of meat. If you look up areas known as blue zones, places where people routinely reach the age of 100, you will see they all eat large amounts of meat.
In my opinion there is a lot of merit to the logical line of questioning behind the paleo movement... i.e. "What is it that humanity evolved to eat?". Science
shows that as a whole we do not digest/assimilate the common dairy and simple carbohydrate products of today that well. Yet if we only go back 150 years ago (which is not that far), and start eating the same foods that our ancestors ate, in the same ways they were obtained, disease will plummet. Today we live longer due to better medicine, but more with more syndromes and disease.
On top of all of this, there is mounting evidence suggesting that our minds and conditioned emotional responses also play a very large role for the creation disease. Showing that there is much more to health than a material reductionist approach.
There are buddhist monks in Japan who are considered to be the worlds strongest athletes. During a certain ritual test monks run 80 kilometres over rough terrain daily for over 1000 days in a row, wearing straw sandals, and getting only two hours of sleep. As well as eating a simple diet consisting of a small bowl of rice and a small bowl of noodles. No athlete on record has been able to keep up with these monks for over a week. To me this shows that there is much more to health than the constituents of a diet. They often mention that if they had even one negative though they would fail the test. There are several free documentaries about these monks online, just youtube search "mount hiei monks".
My main points are these, there are more factors involved with health and athletics than only diet. Its also not just a question of what we eat, but also how what we eat is obtained.