If you look at the number of 3:29 to 3:33 Brits or Americans in the 1980s, you'd have to say that they could compete well with the top Kenyan 1500m runners today. At higher distances, there would be no competition, however. Kenyans and Ethiopians have multiple advantages independently of genetics. They have large numbers of runners training at 'medium' altitude (5000 ft to 8000 ft) high mileage from an early age on soft surfaces (dirt roads), extremely competitive training groups starting in their late teens, numerous examples of success around them, large populations of thin, active people, enormous economic incentives to running, and cultures of running. You need the large groups to produce sufficient numbers of high responders to training to overcome high injury and burnout rates on extremely high quality and quantity training programs.