"Buchanan seems to have a fixation with the Roman Empire. Rome never fell; it just changed it's form of governance"
While there is no single date the western empire 'fell,' there is no doubt that it did whatsoever. The power exercised by the western church, usually from rome except when they had dual popes claiming the papacy, is an entirely different matter than direct colonial administration, taxation, and military control of the vast parts of europe, north africa and asia minor Rome had at one point.
Traditional Rome fell; no doubt, but part of the establishment it started remains within the church. It collects taxes, dictates how people should live their lives and still influences world politics.