"East Africans are crushingly dominant in marathon running." To be more precise, East Africans from East Africa. The effect of biology dissipates in East African communities living abroad.
Yes, if people have other better opportunities then they’re likely to pursue those. I highly doubt they dissipate entirely, if only because biomechanics don’t change. Being tall and skinny with long legs compared to most people will place those of East African ancestry at an advantage for long distance running regardless of culture. As the East African diaspora increases you’ll see more nationalities represented among successful long distance runners but I doubt there’ll be much dispersion in the ancestry of successful long distance runners. The West African diaspora are dominant in sprinting worldwide to a ridiculous extent. I think there are two men ever who don’t fit that profile among those who have run 100m in under ten seconds.
No, but the better the outside options are the less likely people who are athletically gifted are to pursue an athletic career. Think on the margin. There are a lot of people working as quants on Wall Street who would be mathematicians and physicists if there were jobs for them. Likewise as Kenya develops economically a lot of people who could be long distance runners will pursue other opportunities.