Henry George argues in "Progress and Poverty" that latifundia expansion and raising inequality in land ownership was a fundamental cause of the Roman decline:

By the power with which the great attracts the less, small family estates became part of the great estates -- the latifundia -- of enormously rich patricians. The former owners were forced into slave gangs, or became virtual serfs. Others fled to the cities, swelling the ranks of the proletariat, who had nothing to sell but their votes. As a result, population declined, art sank, the intellect weakened, and once splendid civilizations became empty shells.

The hardy virtues born of personal independence died out, while exhaustive agriculture impoverished the soil. At length the barbarians broke through; a civilization once proud was left in ruins. During Rome's grandeur, such a fate would have seemed as impossible as it seems to us now that the Comanches could conquer the United States or Laplanders desolate Europe.

The fundamental cause was tenure of land. On the one hand, denial of the common right to land resulted in decay; on the other, equality gave strength. Every family in the German villages was entitled to an equal share of common land. This impressed a remarkable character on the individual, which explains how small bands of barbarians overran a great empire. Rome perished from "the failure of the crop of men."

As for runaway complexity, I see it as a symptom of gross inadequacy of social, political organizations to changing or critical conditions. When you try to patch multiplying problems with a limited set of orthodox means (and behavior variety and flexibility is suppressed), organization gets desperately complex and blind.