Your analysis is based on statistic numbers, and after this you try to find some hypothetical explanation about statistical results.
This is a big mistake, BECAUSE WR NOTHING HAVE TO DO WITH STATISTIC. Statistic can have a value when you try to explain phenomena with a linear trend, while athletics performances don't follow any mathematic rule.
For example, how can you statistically consider the fact that the same athlete, Ron Clarke, moved the WR in only one attempt from 28'15" to 27'39", with an improvement of 36", when, how you say, in thenext 20 years the total improvement was 22.0 only ?
Following your simple thought, we can say Clarke was clean the first time, and took EPO the next year !
Too many things changed in athletics, and I want to explain some real reason.
1) African athletes didn't exists before 1960. The first
Kenyan finalist in OG was Nyandika Maiyoro, 6th in 5000m in Rome 1960 in 13'51"8, and the journalists at that time were surprised "because black people could only run fast in sprinting events", making confusion between colour of the skin and physiological attitudes. Don't forget that, between the two World Wars, it was common to speak of "negro race" thinking all black people could have the same attitude (that is like to think all the red cars are fast, non depending on the fact can be a low, medium or high powered car).
2) The first track build in Kenya is dated 1957. Who built it was a white English sent by the English Gov. for developing sport in that colonial Country. The name of this man is John Velzian, now Kenyan, and he was also the first coach of Kipchoge Keino.
3) The first Kenyan running Marathon in OG was Philip N'Doo, in Tokyo 1964, finishing behind the 60th position, since there was no culture about long continuous run, on the contrary of Ethiopia, where the main culture was about long continuous run. So, it's not a case that Ethiopians won Marathon for 3 following OG (Abebe Bikila in 1960 / 1964 and Mamo Wolde in 1968), while Kenyans became better on track in shortest distances.
4) Since Athletics was not professional, so it was not possible to consider running a job, the only Kenyan runners belonged to Army Forces, and their preparation started about 2 months only before the main event. Kip Keino told me he could not run on the roads, because people thought he was stupid. "Where do you go", or "Why are you running", without a specific reason : you could run for going to school, for going to work, not running for running, if you did this other people considered you lazy because you didn't do your job (for example, controlling cows).
5) Keino was the first putting in evidence on track during Tokyo 1964. When he bettered the WR of 3000m in 1965, everybody was surprised because he destroyed the WR of Sigfried Hermann from 7'45" to 7'39" (today somebody can explain an improvement like this with EPO...), without pacers and on dirty tracks (and today people speak of doping for athletes running the same time...).
6) In 1972, Ethiopia showed its first top track runner, already 32 years old, Miruts Yifter, going to 27'41" in 10000m with a 4th place that, in my opinion, could be gold without the accident in the last lap. Yifter (whose PB before the race was 29'21") always followed Viren, Puttemans and Gamoudi, about 5 meters behind. At the bell, a Kenyan called Moses was lapped, and kindly went in the second lane for allowing the leading group to stay inside for the sprint. But Moses supposed the group was only of 3 athletes, so after them suddenly went inside, and Yifter had an impact with him, going out of the track. In this situation, he lost about 5", confused, and one judge pushed him on the track again. With all this problem, Yifter finished less than 3" behind Viren (who, on his side, lost time and energies when fell down losing more than 30m and had to recover running faster, about the 7th km. So, no doubt he could already run 27'20" or less).
7) At that time, there were no WCh, only Olympics every 4 years for Ethiopians, and Commonwealth Games for Kenyans the year after Olympics. No money on track, no big meetings (apart competitions in Scandinavia), no motivation. For Ethiopia, there was the boycott of OG 1976, and this means that the best athletes had a period of 8 years without top Championships, and after this again the boycott of OG 1984. For kenyans there was the boycott of Moscow 1980. This means full generations of potentially top runners didn't have any reason to train and to continue their activity.
And of course, if the most talented athletes didn't run, there was little evolution in the WR.
8) When the Italian Primo Nebiolo became President of IAAF, he changed the World of Athletics, moving to a professional system. He created WCh (first edition in 1983, the Italian Alberto Cova winner, without Kenyans or Ethiopians in the leading group), and was the one paying the travel, for the first time, to the Ethiopian Team for competing in World Cross Country Ch in 1982 in Rome (Ethiopia won Juniores Men, with Italy 2nd, and we were not very happy...).