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I will say, though I'll defend this last AoT arc any day because I think it's a valid message that takes the story's thematic thesis full circle, moral ambiguity and harsh realism still has a place in the story, and I don't mean to pretend it doesn't exist. That being said,
I don't think this last arc is as black and white as some people will try to have it. This fact also obviously depends on what kind of conclusion we reach but Eren's motivations for one are very nuanced, and each of the characters involved have a different reason for fighting.
The only real idealist is Armin, but he's always been that, he's been a purveyor of hope and empathy and takes an opposing stand to Eren on basis of consistent traits shown in the story. Plus I don't think it's wrong for the story to take a stance against "destroying the world".
That doesn't make it a 2-dimensional story about good vanquishing evil & live happily ever after. Yes Marley was used to show the opposing perspective, and make the world & morals more grey & nuanced,
but that was all so that we could reach this point so "the world" is something that is cruel yes, and especially towards the people of Paradis, and therefore there is validity in Eren declaring war against it, but there's also reason to want to defend it, as we saw in Marley.
In other words it's back to this. The same reason Mikasa found her resolve to live after she thought Eren died is the same reason they fight against him now, and why Mikasa is emphasized so much in these last few chapters.
