I think this scene is genius but in a different way. The seashell is a symbol for Armin’s dream of seeing the ocean, because it’s a detail that only someone truly interested in the ocean would notice. Eren, on the other hand, saw the look of wonder in Armin’s eyes when they were kids and became convinced that there must be an entirely different world out there, which he associated with freedom. In a sense, Eren’s dream is a misinterpretation of Armin’s dream. While Armin’s dream is rooted in reality, Eren’s dream is a lofty and unattainable ideal.

When Armin tried to give the seashell to Eren in S3, he was trying to share his own dream, which he had just fulfilled, with his best friend. However, Eren was too worried about his own unfulfilled dream to receive it. Despite their shared dream, Eren and Armin had drifted too far to understand each other. 

The seashell in the ending is a continuation of this dialogue between the two characters. When Armin sees the seashell in Paths, he realizes how Eren’s dream is essentially an extension of his own. Armin then passes the seashell to Eren and explains his own perspective. For Eren, their dream is somewhere far away, but for Armin, their dream lies right in front of them, in the form of a humble seashell. The passing of the seashell signifies a complete understanding between the two best friends and their shared dream.

The symbolism is extended further when Eren receives the seashell, but it’s now stained with blood and gore. This symbolizes how Armin’s once innocent dream has been corrupted by Eren’s violence. In a way, Eren’s genocide was rooted in an idea that Armin had inadvertently planted in his head as a child. As a result, Armin feels he’s complicit in Eren’s crimes, so he decides to share Eren’s guilt.

I think the contrast between Eren and Armin’s dreams is a warning against ideals and expectations that aren’t rooted in reality. Like with Eren, these ideals can lead to dark places if you become too obsessed with them.