I think it's pretty clear that part 1 was Stephenson's "how would modern humanity really react to an apocalyptic scenario?" answer, and part 2 was his "I wanna

I think it's pretty clear that part 1 was Stephenson's "how would modern humanity really react to an apocalyptic scenario?" answer, and part 2 was his "I wanna write about theoretical orbital megastructures and epigenetics in a near-future sci-fi epic!" statement. The fact that the two parts go together as well as they do is, I think, impressive.