"The Roman Emperor" a new book by Mary Beard opened my eyes to one thing, the idea of the job simply being too big for one person, regardless of the era. She notes in the book that the emperor's job and the city of Rome itself changed very little in the first 300 years CE. That's way too much stasis. Progress and change are twinned as are stasis and decline. The answer to why the decline can't be found in economics or sociology unless we also focus on the structure of governance.