The following males were junior medalists between 1986 and 2002, mixed among the Gebs, Mezegabus and Woldes:
Peter Chumba, Henry Kirui, Abreham Assefa, Regasa Habte, Giuliano Battocletti, Kipchumba Mitei and Aaron Gabonewe, Francesco Bennici. Heard of any of them?
How about these women?
Cleopatra Palacian ROM
Ann Mwangi KEN
Simona Staicu ROM
Philippa Mason GBR
Yimenashu Taye ETH
Edna Kiplagat KEN
Etaferahu Tarekegne ETH
Yvonne Lichtenfeld GDR
Or put another way, I'd say it's fair to say that just 9 of the 22 medalists in the girls 3k, for example, from 1986-2000, have become senior stars. Look at the list (http://gbrathletics.com/ic/wj.htm) and see if you can argue differently. I'd say it's fairly accurate to say that in any country, over time, about 30-45 percent of female junior stars become senior stars. The percentage is probably a little higher for males, maybe 45-60 percent.
The point is, only a percentage of junior stars turn into senior stars in Kenyan and European countries, too! It's not just a US problem; it's the way it is.