At a certain point, every population runs into the Secretariat phenomenon. I.e. For any given population you can recreate the ideal genetic makeup for a certain task but you cannot exceed it. Genetic engineering, giving humans kangaroo tendons, or once in an epoch random mutations are obvious exceptions to this rule but the point stands for all practical purposes.
Matt Centrowitz, Jr is the offspring of a world class miler and a national class sprinter. His genetic makeup and nearly ideal training background have enabled him to become an Olympic champion. He might someday break the AR for the 1500 or mile. But he is is certainly not going to shatter the WR by six seconds, the equivalent of dropping the marathon record by three minutes.
Kip Keino and Jim Ryun's kids were to good to superb runners but they were not all-time. The same will most likely be true for Haile Gebresalassie and Keninesse Bekele, regardless of who they have kids with.