Board Thread:Manga/@comment-27321453-20190908104148/@comment-27125793-20190922232329

As TKG already said, Eren only saw glimpses of the future. He probably did not know Sasha would die, for example. While the final result might have been set in stone (though only Eren and Grisha knew that, and not for long), the details of how it is reached were unknown to Eren. Plus, it still depends on the actions of the characters, especially Eren, and their abilities. If the Attack Titan did not have future sight, it's more than likely that the final result would be very different. All of that combined means that Eren still needed to struggle, not just physically, but emotionally and morally. While the broad idea of how to achieve it was fairly certain (Eren and Zeke coming together), the details were not known. That's why Eren objected to the whole Historia becoming a broodmare idea, it's why his resolve faltered at the idea of killing innocents in Liberio, and why he cried when Sasha died. He could still control all these relatively minor details to the best of his abilities, make as few sacrifices as possible, while having to make sure to not stray off the path that leads to the future he saw, and dealing with the immense pressure that comes with it all.

Can you imagine it? Being the only one to know this future, which seems failry bright from his perspective, but also knowng it was only possible through firm actions and choices. That HOW it is reached was still a rather flexible affair with many possibilities, and it was up to him to minimize the damage and sacrifices, that the fate of many, MANY people was in his hands, all the while making sure that it all leads to that worthwhile future. That is a pressure of nigh-inconcievable proportions.

Knowingthe future result only made things harder for Eren. While it told him that the overall result was good (in his opinion), reaching still required action and minor results (delaying Marley, obtaining Zeke, etc.), which could never be achieved without dirtying his hands. Anytime he had to make a hard decision, he struggled, wondering if the future he saw was worth it. There was probably a part of him that believed that not acting would change that future, and that a different result was possible, before realizing it's the only road ahead. And even then, that future that he saw and thinks is worth all the sacrifice is only possible because he was able to see it in the first place and could make pragmatic decisions based on that vision; in other words, no Attack Titan power, no future that Eren saw.

If you want me to give you the best example of why it's not necessarily a bad plot device (at least in terms of influencing the past), it would be Harry Potter and the Prisoner of Azkaban. It shows that the good results at the end were only possible because Hermione had a time-turner. If that item had not existed, or at least been in Hermione's possesion, Buckbeak would be dead, so would Harry and Sirius. The Attack Titan functions in a similar manner... ONLY they cannot physically interact with the world in the past but only Grisha's conciousness and influence his actions by giving him memories of the future, meaning that everything Eren is achieving with the AT's power, a good chunk of it he's achieving indirectly through the actions of others, without the more surefire way of doing it with his own hands. He is more limited than Harry and Hermione were.