There is no question that the Roman Empire reached its peak under the “five good emperors,” Peter Turchin explains:
There are literally dozens of quantitative measures for imperial might that all agree with each other: territorial extent, overall population, internal peace and political stability, economic activity proxied by shipwrecks and the amount of industrial pollution, monument building, production of literature and art … After the death of the last “good emperor” in 180 all these indicators headed south. Together they tell us a much more quantitative and nuanced history than an artificial binary construct of “the Fall of Rome”. As a single example, here’s the trajectory of the volume of imports of particularly fine ceramics from Africa to Italy:
If we follow these trajectories, we will learn that there were peaks and valleys. For example, a key indicator, social and political instability, went up after 180 and stayed high to the end of the third century. However, there were several peaks on top of this elevated level, recurring at roughly 50-year intervals. Such dynamical richness doesn’t fit the narrative of a “collapse.”
Most of the fourth century was relatively peaceful, but then the western half really disintegrated. The center of gravity moved east, to Byzantium, which experienced its own decline in the seventh century. Which was followed up by more cycles.
Thus, a much better question is not why Rome collapsed, but why the Roman Empire experienced those massive waves of social and political instability, accompanied by political fragmentation, population decline, and (later) dramatic loss of literacy, disappearance of monumental buildings, decrease of economic activity etc.
Turchin, of course, explains this through his structural-demographic theory:
Growing political instability is first and foremost a result of elite overproduction leading to excessive intra-elite competition and conflict. This main driver is supplemented by mass mobilization of non-elites resulting from popular immiseration and by failing fiscal health of the state.