I do not think it is proven that you have to run your easy runs easy. It is not as simple as saying there is a certain pace that is optimal. Aerobic endurance is developed well when running at or below the threshold. Then there is a benefit in running so long that you deplete the glucogen storage (after 60-90 min) and have to resort to faster muscle fibres and fat metabolism. running well under threshold also trains the fat metabolism in general which is important. Then the time one can do this aerobic running is important. If you run close to threshold, it is limited how long you can run and doing easy pace can give way more time for important endurance adaptations such as capillarisation and mithocondrium development in slow twitch muscles. A final argument is that we need to do both volume and quality and if you run the volume too fast, there is not enough recovery and energy left for the fast workouts.
So I think we have a large zone below threshold that is good at developing the endurance, but it needs to work in combination with the fast workouts and general recovery.
So one can develop doing moderate running and little easy running, but it might be hard to recover and attain high enough milage. On the other side, if one is able to recover from still doing less easy, then fine, nothing to regret.