It did feel like the last part was Stephenson deciding that he had far too many ideas about how this would all turn out in the end to leave it that way, or even with a short epilogue, so he expanded it into an extended epilogue, which is what it feels like in the end. Add in him wanting to throw in what he thinks are some cool technologies, or how weapons would develop when carried through an arc that largely bypasses gunpowder, or when satellites are ubiquitous, and you get the filler segments we ended up with.

I largely enjoyed that part, but not as much as the first two parts. I would have been supremely annoyed if he didn't give some explanation of what happened after the first two parts, since he didn't really resolve humanity's problems as much as trade one problem for another. Some sort of extended epilogue was warranted, and I can see how deciding just the right length for it could be hard to get right when it needs to relay a lot of information to satisfy the readers. What we got was the third act which doled out the history sparingly but consistently over its length, but the story was lacking.

Yeah, it's not great, but it's at least back to being kind of uplifting sci-fi. I would have been happier with a book that was a more developed 'future world' with a few flashbacks to the big disaster, rather than dwelling on it. If I want to contemplate death and destruction, there's plenty of that in the real world.