A crucial turning point in that earlier history occurred when men and women of good will turned aside from the task of shoring up the Roman imperium and ceased to identify the continuation of civility and moral community with the maintenance of that imperium. What they set themselves to achieve instead . . . was the construction of new forms of community within which the moral life could be sustained so that both morality and civility might survive the coming ages of barbarism and darkness. If my account of our moral condition is correct, we ought also to conclude that for some time now we too have reached that turning point. . . . This time however the barbarians are not waiting beyond the frontiers; they have already been governing us for quite some time. And it is our lack of consciousness of this that constitutes part of our predicament. We are waiting not for a Godot, but for another—doubtless quite different—St. Benedict.This quote strikes me as interesting in its historical perspective, in that I'm not sure that it actually reflects how the people of Late Antiquity and the so called Dark Ages thought about Rome.
There are various points we can look to as the mileposts along the Fall of Rome: The Visigoth's sacked Rome in 410 AD, marking the first time the city was entered by a foreign, hostile army in almost eight hundred years, since the sack of the city by the Gauls in 387 BC. The Vandals looted the city in 455 AD, though due to a prior agreement with the pope they mostly avoiding burning buildings or killing people. In 476 AD, the last Western Roman Emperor, aged only about 16, was deposed by the Germanic general Flavius Odoacer in 476, after which Odoacer declared himself King of Italy. (If you want to place the original Benedict Option in relation to these events, Benedict was born around 480 AD and founded his monastery at Monte Cassino around 529 AD.
However, even this sixty-six year range from the first sack of Rome to the deposition of the last emperor doesn't really capture the period when in MacIntyre's words "men and women of good will turned aside from the task of shoring up the Roman imperium and ceased to identify the continuation of civility and moral community with the maintenance of that imperium". The Roman emperors in the East continued to be actively interested in ruling the West. Justinian waged a series of wars for control of Italy from 535-554 AD. More locally, the Senate of Rome remained active as an institution within the city of Rome even after the last emperor was long gone, though it gradually dropped into obscurity. Its last recorded act as a body was to acclaim a pair of statues to the Eastern Emperor and Empress in 603 AD. In 630 AD, the senate building was turned into a church by Pope Honorius I, who had first got permission from Emperor Heraclius in Constantanople.
As the fact that Pope Honorius first consulted the emperor before taking over the senate building indicates, even at this very late date the Roman empire as a legal and governing authority had sway (even if its power did not effectively spread to the successor kingdoms of the West.) Nor did that cease. When Pope Leo III crowned Charlemagne as Holy Roman Emperor, he wasn't just re-instituting an office of Western Emperor that had been vacant for three hundred years. The Roman Empire which was still in existence (and still seen as an overarching world authority) was in the midst of a succession crisis of sorts. Irene of Athens, who had been the wife of Emperor Leo IV, and was mother to Constantine VI in whose name she ruled for some years before having him killed after a coup attempt. Charlemagne had actually entered into negotiations to marry Irene and combine his own empire (which spanned Western Europe and included Italy, France, Germany, and a sliver of Spain) with the Eastern Empire, which at this point included Turkey, Greece, Sicily, and Sardinia. Crowning Charlemagne as Roman Emperor not only pointed to the fact that even in 800 AD Rome was still seen as the symbol of law and stability in Europe, but also that Irene (who had recently had her son the emperor killed) was seen as a semi-illegitimate ruler for the Roman Empire whom the pope might like to see replaced by his successful Frankish ally. Who was Roman Emperor still mattered in the West even though it had not been under effective Roman control for three centuries.
I think it's worth asking: If people had not yet fully turned aside from the idea of Rome by 800, does it make any sense to talk about the fall of Rome and the transformation of Western culture into the Christendom of the Middle Ages and beyond as a matter of people turning away from the idea of at all? Rather, in the way that contemporaries talked about the empire and its importance, Rome retained a relevance long after it was practically gone. The idea of the Benedict Option is that people should recognize that the mainstream culture is debased and turn away from it to form their own small communities, out of which seeds a new civilization might someday arise. However, this is not how people at the time of Benedict thought. Monastics went into the hills and secluded themselves as monasteries not in order to plant the seeds for the abbey towns of the Middle Ages, but because they wanted to seclude themselves in a life of prayer away from active involvement with the world. Yet when people considered politics and culture they considered Rome and the Empire to be relevant long after it had ceased to exist for practical purposes in the West. Rather than people turning away from Rome to build something else, the idea of Rome remained for centuries as a symbol of what people wanted to return to. Charlemagne took up the name of the Caesars. Roman law continued to be actively referred to both by civil and church authorities. Rather than people turning away from Rome, their ideas about Rome shaped the world which they gradually rebuilt after the periods of political, military, and economic chaos marked by the early barbarian kingdoms.
What does this tell us about how to live as Christians in our modern, increasing post-Christian world? I don't have a pat answer to that question, which is why I'm not hawking a book on the topic. But I do think that it's very much worth noting that the post-Renaissance and post-Enlightenment ideas of the 'Fall of Rome' are to a great extent the result of those eras rejecting the claims of direct descent from Rome which those in the Middle Ages still made. People did not give up on Rome. It withered away even as they continued to profess attachment to it.