That's the problem with letting characters act how they would actually act, and make decisions that make sense for them. Those decisions often don't end in a way that's concise and brings closure.
I think he tried to sidestep this issue quite a bit in Seveneves in a fairly clever way. To me personally (and I know there are others who disagree), the third act felt the weakest. Despite that it did bring closure to the story. There was no finality to the characters themselves - it skipped over all of that, but there was closure to the story.
One thing I'm curious on the other end of the spectrum - who does HN think writes good endings to complex scifi stories?