Lots of misconceptions here.
What makes people faster are primarily two things:
1) HARD workouts.
2) RACES.
Everything else is just supplementary.
Yes, elites run 80-90% of their mileage "easy". Why do they do that? Not because the easy miles are the best training stimulus to become world champions in the 1500, 5k or 10k. It's because it's literally the only thing they can do in order to fully ABSORB the workouts and BENEFIT from them.
If they would run any faster than what is easy for them (say they would do all their easy runs at 4:40 pace), there is no way they would still be able to HIT their key workouts as well as they could with truly easy runs, or even if they could still hit the workouts, they would not be able to recover anymore and get faster.
I know lots of people who changed from a 2 workout a week schedule to "just easy runs" and stagnated or even got worse. You don't see many elites doing that, like cutting their weekly workouts so they could run either MORE miles or FASTER miles without workouts.
Kenyans running very easy on their easy days is just another proof that workouts are the most important thing - they just hit the workouts so hard, that they have to run even easier on their easy days to absorb them, recover and be at full strength again for the next workout.