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I don't think the ending was intended to act as a climax to the narrative. Rather, it's the logical conclusion of several ideas shown earlier in the story; modifications to how you think that can't be reversed; alien consciousnesses that you can never actually communicate with; the logical conclusion of the original quest to prolong humanity's simulated existence, which is why they go exploring in the first place.

I wonder if it's just common to have many unexplored ideas and unfinished side stories? I recall Accelerando also had many of these, probably A Deepness in the Sky as well (though I don't recall it in detail anymore).

Or maybe I just recall noticing that detail and forget about the ones where I didn't notice it.

That was absolutely my feeling as well.