The third part of the book felt like it was meant to be a full-length sequel to Seveneves, but was cut down to a novella-length post-script.
(Which to me still feels like the last third - to me, the book feels like the book has three sections: the setup, the survival, and the aftermath. I haven't read it in long enough to see if it's actually split that way in any sort of specific "Part 1, Part 2, Part 3" way.)
More authors should be brave and have their stories end bleakly.
Great examples that come to mind would be 'Death's End', 'Echopraxia' (to a lesser extent also the prior 'Blindsight'), 'Consider Phlebas', just to name a few.
And I dunno about age improving it. Niven and Pournelle’s politics just looks kind of ridiculous now, even if you don’t know exactly who they had a bee in their bonnet about.
I don't think that's horribly unrealistic, is it? He's pretty much a scientist the media picked to represent the topic, like Dr. Fauci in real life. They could have picked a coronavirologist, but they didn't.