John Robb at Global Guerillas has some speculation about the rise and fall of
civilizations. The truth is that we know so little about the ancient Roman world, its politics and its economics, that we are more likely to simply project our own obsessions and preoccupations on their history, as we are to gain genuine insight from their example. But, it is still fun to speculate and draw parallels.
Like anything organic, I suppose that civilizations can be said to have something like a natural lifecycle: birth, growth and death. The ancient Hellenic world lasted about 1200-1300 years or so, from its emergence at the end of the Dark Age that followed the collapse of Bronze Age civilization, around 750 BC, down to the Fall of Rome in 476 AD, or, if you prefer, the Plague of Justinian in 541-542 AD.
As others have noted the Eastern Empire went on, though, there were severe strains there as well -- severe enough to mark out a Dark Age of at least a couple of centuries, even in the East, even where the institutional Empire and the City of Constantinople appeared to survive.
I find the handwaving over an opaque "complexity", as well as the analytically empty complaints about high taxes and a debased currency to be unsatisfying.
The early city-state culture of trading cities, and of Rome's Empire, was a hungry and expanding beast, bringing new lands under cultivation and extending the scope of peaceful trade throughout the Mediterranean basin and beyond. There must have been gains to economic productivity from the Magna Gracia project and the Carthaginian expansion into Spain, as well as Rome's methodical conquest of Italy and its founding of new cities.
Solon's reforms in Athens, planting olive groves, etc., must have unleashed energies comparable to those released by the French Revolution, energies, which would have been capped and finally extinguished by the short-sighted plutocracy of Rome and its latifundia. Reducing Sicily from the breadbasket of Rome to a wasteland must have required centuries of poor farming practice, soil erosion, pestilence, famine and plague.
The ancient Romans, in the latter days, were not technically inventive. They never seemed to grasp the potential of windmills. They failed to invent the stirrup or a workable horse collar or really good heavy plow. The Empire was big enough to afford the kind specialization of labor, which might yield technical advance, to meliorate the shortage of new slaves and declining yield of soils under cultivation. Instead domination by the rich of the poor masses seems to have deepened to the point that the poor were liable to extinction in plagues and famines, while the competition for power among the elites became ever more lethal.
Our modern civilization emerged from the Dark Ages around 800 or 900 AD, with a fusion of the Franks and Vikings, and the invention of the motte and bailey castle. A predatory military caste began an expansion that ended in something close to world conquest by the end of the 19th century. Accelerating technological advance has marked its character, but it is worth remembering that the predatory character of the elite, at this culture's core, was there from the beginning. William the Conqueror, the Angevin Henry II, the Crusaders who sacked Constantinople -- these were ruthless, violent bandits, whose first order of business was to exploit the mass of farmers everywhere, from their walled castles.
The normal state of Western states over the centuries has been a low-performance equilibrium, in which oppression and domination by parasitic elites was often just short of collapsing the society. France in the 17th century was the richest and most powerful country in the world, seemingly on the verge of ruling North America, India and much of Europe, but by the end of the reign of the Sun King, the country lay prostrate. It would struggle on fitfully for another 75 years, before its feudal parasites were overthrown. The outburst of egalitarian energy would catapult France back into contention for the domination of Europe, if not the world, before the forces of reaction again gained the upper hand. Feudal Empire would dominate much Europe right up to the conflagration of World War I, in the 20th century.
That we are again seeing a parasitic elite emerge, in a neo-feudal order, should not be seen as anything but a renewed expression of Western Civilization's DNA.