This book has an implicit public choice theory, which I read as follows. Division of powers is ultimately impossible, so there is either rule by one man or rule by the crowd, or as it will turn out, both at the same time. Augustus represented the perfect fulfillment of the Roman ideal of statehood, and thus his reign heralded the beginning of the end. Augustus had replaced all institutions with his perfect yet ultimately destructive personhood. Given the importance of fortune, rule by a single man then meant an eventual downward ratchet in the quality of rule. Tiberius, Caligula, and Nero were successively worse, but even a lucky break in the genetic lottery would only have put off this trend; there is more downside than upside in the weak-institutions version of this game. The insane Nero plays all the different Roman roles on the stage and metaphorically Nero becomes Rome. He plays to the crowd, and the suicide of Nero is the suicide of Rome itself. Hope lies in the civilizations of the less historically conscious barbarians, who live on the fringe of the story, never becoming memorable but the modern reader knows they will allow Europe to one day live again.
For related points, I am grateful to fellow participants at a Liberty Fund conference this last weekend.