Since you mentioned the Olympics, I counted medals from male competition in the 800m-marathon, not counting the steeple (which Kenya obviously does fantastic at and actually dominates), from 1960 through 2016.
There were 5 events, 800/1500/5000/10000/marathon, with three medals each per Olympics for 15 Olympics.
That yields 225 total medals, not counting any ties. Kenya had a fine total of 43 medals, Ethiopia 25. I might be off one or two and I did not count Paul Chelimo or Wilson Kipketer or other former Kenyans.
Kenya and Ethiopia have done very well, but they have in no way dominated the Olympics for 50 years.
Kenya medaled only three times in the 800 and twice in the 1500 prior to 1988, while Ethiopia has never medaled in either the 800 or 1500 (men). In 1988, Kenya medaled in the 800, 1500, 5000 (all golds) and won bronze in the 10000 and silver in the marathon. That was the beginning of really striking consistent performances for Kenya (1968 at altitude is an outlier here with five medals for Kenya from 800 through 10000).
Ethiopia won golds in three straight Olympic marathons from 1960 to 1968 and bronze in 1972then there was a 28 year drought before they medaled twice in 2000. They also medaled once each in 2008 and 2016.
The 5000 and 10000 have been Ethiopia's best event but their first medal was in 1980, golds for Yifter in both. They did not medal in the 5000 again until 1992, missed out in 1996, and then won gold in 2000, silver in 2004, gold in 2008, silver in 2012, and bronze in 2016. In the 1000, Ethiopia won nothing until 1968 (silver), bronze in 1972, gold and bronze in 1980, nothing in 1984 or 1988, then bronze in 1992, golds in 1996 and 2000 (plus a bronze in the latter), then gold and silver in 2004 and 2008, bronze in 2012 and bronze in 2016.
Excellence, and selective at that, but not dominance.