The primary flaw with the writing of the 8UE situation is that the writers didn't go hard enough. The timeline-overwriting plan needed to hinge around the idea that there simply was no stopping or undoing the impact of Black Rose – it needed to not be a destructive historical event but an ongoing process that would freeze the whole world and spread to the shards as well, so they rushed to give the people of the past a second chance before all was irreversibly lost.

Unfortunately, they didn't do that. They held back and then tried to undo it altogether with An Unpromised Future. They confused their own storywriting and undermined their own point.

They tried to be less brutal and actually made it worse.

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Meanwhile, I go into our visits to the past with the prospect of the time loop first and foremost. Yes, it would be nice to have the freedom to tell the whole truth and save the ancients from their fate, but that is inherently impossible. There is no mere "planting the seed to save their future generations" and then retreating to your own timeline – if you've changed it, you've changed it, and still not actually undone the suffering that has already happened.

In fact, given that we cannot undo the suffering experienced so far by the ancients in this timeline, then I would argue that the best thing we can do for them is to make sure we stick around in this timeline to bring about a happy ending to what they were trying to achieve. If we vanish off into a second timeline and abandon this one, all that they went through up to now was for nothing.