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This is just me trying to make sense of how the chapter title of 139 is connected to the extra pages so this might come off as more of a stream of consciousness than an organized post.
The first thing I noted was that the tree is constantly growing admist the growth/development of Eldia and its subsequent fall and destruction. I'm not sure whether there's any intent behind it, but from what we know this is the site where Eren was buried and it may be interpreted as the fruits of Eren's actions being displayed. In the first panel the leaves are barely covering the tree and it may be seen as the long lives of Eren's friends being the first fruits of his actions. They all live long lives with their families and grow to old age all the while keeping him in their hearts as exemplified with Mikasa.
Next, we see War in Heaven sorry I couldn't resist it. The tree grows a bit more and the seeds of Eren's actions are shown more exhaustively. His actions have brought about war, a continuation of the cycle and ultimately, Paradis' destruction. All the good has faded away and the development has crumbled. The Eternal Return, Moira, Ananke, Fate and the Cycle are bound to the world. It almost serves to validate Reiner in 117 and Zeke from 137. Striving is futile, and despite the efforts of Eren and the Alliance, this is the destiny of the world.
Then there are the final pages.
I think there's something quite hopeful about them. Despite the fact that the story ends with the grievance of a child wandering in the forest, seemingly destined to repeat the titanic tragedy from the ages before, there's also a sort of hope.
This child doesn't seem injured or depraved like Ymir was, he seems curious and almost in awe. And I think this rekindled the spirit with which the Survey Corps faces the cruelty of the world. If we don't know we'll see, if they don't know us, we'll show ourselves to them.
So though things are left slightly ambiguous, the parallel to Ymir serves to show us that this boy's Will will shape the new age, our will in the face of cruelty will shape the direction of the world. So as Nicolo said, though this devil may lie within the heart of humanity, we still need to strive our hardest to leave this forest of our making.