If I were king the rule would be: not eligible to compete for a country until you have been a citizen for four or five years.
Lets see... athlete comes to USA for college 17-21... athlete graduates at 21... takes 3-5 years to gain citizenships (25ish at this point) and another four or five years under your law puts the runner at 30. Great idea. Waste your prime trying to become an American....
Meanwhile, every American sport besides track & field is 100% cool with immigrants.