Despite all my issues with OC3 and how it wrapped Extella up, it does do something that I only now appreciate:
It vindicates Twice and his idea of saving the world. In the end, not even Hakuno was able to veer humanity from the dead end it was heading, even after becoming Sovereign and defeating the Umbral Star. Humanity continued to stagnate and ultimately lost it's position as the Prime Species to another race and while they were able to resolve the matter peacefully, it still means after everything humanity has been through, everything that it has done to get to this point, it still lost in the end.
It's as what was said in the materials of Extra: Humanity in the World of Extra no longer wished for a future, only the continuation of today and, as much as I love Extella Hakuno, they never did anything to address that issue. They opened up SERAPH for humanity but that alone wouldn't fix the mindset humanity had. If anything, it'd just make it worse due to how much better things would be in SERAPH and since Extella Hakuno, similar to Last Encore Hakuno now that I think about it, never received the affirmation that they were human from Rin or Rani due to having saved neither one in Extella, it feels like the end result would be Hakuno taking a similar position Twice did in maintaining SERAPH and protecting it but not trying to do anything for humanity. After all as she said in both Last Encore and in the materials of Extra and CCC, she also viewed herself as a ghost from the past who didn't have the right to dictate the future for the people living in the present.