In 428 A.D., Prof Traina, of the University of Rouen, takes on the question of the proverbial "Fall or Rome."  Essentially a survey of the state of the Empire in A.D. 428, a year which would generally be considered well into the era of the "fall", 428, Traina looks at the condition of the various regions of the Empire, as well as the strength of the economy, and the state of the armed forces. He concludes that in 428 the "inevitability" of a "Fall" was hardly obvious even in retrospect, despite previous scholarship. 

Although some parts of the Empire had been occupied by so-called barbarian peoples, this was usually with some degree of Roman agreement, and the new-comers were often happy to become allies and subjects of the Empire, and willing to undergo some assimilation.  When adequately led, Roman armies were still usually able to beat all comers, as they would demonstrate as late as 450, with the defeat of the Huns, in cooperation with allied "barbarian" contingents. 

The key issue in the problem of the survival of the Empire was, in Traina's view, was leadership, not lead poisoning or barbarization or Christianity.  The imperial system was heavily dependent on the Emperor, and a series of ineffective emperors, among them several minors, plus an ongoing problem with usurpation, was what really doomed the Empire, at least in the West.

A good book for anyone with an interest in the Roman Empire , Late Antiquity, and the rise and fall of empires.