That could also explain why the elder races seems to be vanishing during the fourth age. It's not that they are extinct, it'just that they are hidden in those dark corners of the Great Forest, out of reach for humankind. I love that survival through compromise-between-old-ennemies idea because it just goes so well with my own angle on the 4th Age.
In My Glorantha, the fourth age is when the Elder Races go into hiding, much like the Gods had to do during the Great Compromise. For the first 3 Ages since the birth of Time, gods could only be heard via ceremonies, and could only be reached by heroes. But their creation, the Elder Races, were still living near humankind pretty much everywhere on Glorantha.
In the 4th age, the Gods are completely gone, but, IMG, the Elder Races are just very very rare and hard to reach, much like the Gods used to be.
It's much like if there were only 3 ages, and that this 4th Age would in fact be the 1st Age of a brand new world, born out of New Compromises between the Elder Races.
Using the Great Dark Forest unifying Trolls and Elves as an inspiration, I want to discover what the other New Compromises could be. Maybe the Trolls had a New Compromise with the Dwarves too, allowing them to hide under the Dark Stone Bridge.
Maybe the Dwarves and the Elves have their own New Compromises, allowing them both to hide in the depths of the Branching Caves or in the canopy of some Growing Ruins.
Maybe the participation of long gone Elder Races would become essential for some of those New Compromises, and that could further explain the return of Pinchining the last Gold Wheel Dancer. (Who would have been the Gold Wheel Dancers' old ennemies ?)
Maybe some lesser known Elder Race would have annihilated its old enemy long ago, and without any means to bring them back and accept a New Compromise, their own fate would be sealed.
Maybe this 4th Age is a New Compromise between Humankind and the Gods, allowing those older powers, those runic essences, to hide inside us, in our own heart and soul, and manifest themselves in our passions and our moments of transcendance.