Board Thread:Questions and Answers/@comment-4929778-20160415205315/@comment-7630054-20190109190840

A simple personal theory that I'd not want to turn out true since it's pretty boring one: the story will probably end in something apocalyptic (if not for the world, then at least for Eldians), so Armin isn't so much as narrating the story but sending his memories of it to Ymir Fritz in the past (maybe that is what 'to you, in 2000 years' refers to), so she can see what her playing with forces she can't comprehend will bring.

That implies Armin inherited the entire set of Titans to achieve that, and Ymir will not touch whatever turned her into Titan after seeing the memories. If so, the timeline changes and 'you're free' refers to Eldians as a whole, since, honestly, I don't see any other kind of 'happy ending' achieved in this world, it's either Eldians utterly eradicated or history altered so they never were able to turn into Titans at all (and the world is unaware, so they won't have a reason to murder them for 'sins of the past').