History of science books thankfully stave off that final third until at least 80%. However, their final chapter or two universally manages to be a letdown. It's either wild optimistic speculation, hype for a theory that's debunked 5 years after publication, or a focus that accidentally happened to predict the course of science post-publication. The story is told in a tonally jarring manner compared to the tight narrative in the rest of the book.

My #1 suspect for this disease is a desire to connect the content of the book to real life. Such attempts miss more often than they drive the point home, even if they're factually correct.