Part of Rome's downfall was they were bridged between tribal and state government and their inability to evolve new forms doomed them to a system unable to meet unreconized challenges. The tribalism resulted in a lot of foolishness such as hereditary tax collectors and also reversion to cultic emperor after having dabbled in a republican form of governance. Its just too easy to invest power in a family that has proven itself competent as rulers.

(We often have a dalliance with that seduction in this country. Virginia's Lee family is an excellent example of landed aristocrats who's love of country and sense of duty helped guide our country for nearly 100 years. Someday somebody will write a semi-fictional account of them that will be the American version of the Greek Atreides. The Adams family and the Murthas, were others. The Roosevelts, Kennedys and Bushes are modern examples of the same phenomenon. I don't know why this country has never embraced royalty. Its a character defect, I guess.)

The main thing that held from the Romans from adaptation to emerging circumstances was their conservatism. Their resistence to adaptation was such that they'd kick-a$$ rather than change. That worked great so long as citizens were willing to fight but failed when they became too dependent on foreign armies for Rome's defense. Also, their conservatism prevented them from evolving beyond reliance on a conquest-based, slave society.Everything in their lives was institutionalized by religion, including the state, which invested conservatism with the power of deity.

One take-home message is, IMHO, a country that is overly reliant on conservatism dooms itself.