Came here to say this. Both Seveneves and Fall have long unending psuedo-epilogues which add little to the main story, but also don't really wrap anything up satisfactorily either.
My issue with Seveneves is the first part (two parts I guess) was a pretty first-rate thriller then hard to believe transition to equally unlikely and not very interesting IMO outcomes which, as you say, weren't really wrapped up either.
I think the disconnect here is that I get the impression he wrote the entire first part just to justify writing his "SF with multiple humanoid species that all have 'hats' but there's a good reason for it" story which forms the second part. (Part of the problem being that... there's really not a good reason for it - Stephenson's grasp of biology is much worse than his grasp of computing technology.)
Oh I pretty much agree. There's a good argument that the whole first part of the book was to get him to a point where he could write the story he wanted to write. But, if so, there were probably better ways of getting to that point.
And, even if you accept how they got there, the last part of the book just wasn't very compelling for me.
I kind of liked how openly seveneves seemed to be about building some fantasy world for stories (or perhaps even for some video game or p&p rpg, with a number of clearly cut out character classes?), then getting gloriously sidetracked in the background explanation story and finally accepting that the world just built isn't remotely as interesting as that background explanation story. It's as if at some point half way through he realized just how much he was being "typical Neil Stephenson" and decided to go with it, to ten-up himself.
That last part almost seems like a form of trolling, the smallest viable story to claim "I had you on a sidetrack all the time and you did not even notice"