Peruse the results of the top ten men from the Falmouths of the 80's to today.
Times have remained consistent. Americans have disappeared.
Certainly the Kenyan athletes who run the roads of Falmouth today are not so superior to the Americans of yester-year.
I think you have on those rose-colored glasses. Your first statement is clearly false. Times have not remained consistent; they have gotten faster, especially when you look on average. American top-ten performances have disappeared, but this has coincided with an improvement in the average race time (top 10, 5, or 3 in a given race). Time-wise, the Kenyans who run today *are* superior to Americans of yester-year. On average, about 15-20 seconds faster, even including their "slow" years (like 2002). You can stick with the "Americans have disappeared" and still be correct, but to pretend that the Kenyan competition hasn't improved the quality of the race is wrong.
The fastest of the "peak American" years (1985) is almost 25 seconds slower than the fastest "peak Kenyan" year (2000). And there were only 3 Americans in the top ten in 1985. Maybe you meant "white guys", I don't know. If you look at the *fastest* year with more than 5 Americans in the top ten (1984, with 7), it's slower than NINE other years with 0, 1, or 2 Americans in the top ten. And, three times out of five, that American is Khalid Khannouchi.
I'm trying not to practice that "number thing" without a license, but the results do not support your conclusions.