Sorry, I can't agree with nearly anything you said in your post. I've decided to quote only this part because in my opinion it is a huge mistake and lack of understanding of the importance of culture and religion in historical process. The "state" is much more than just the assembly of different official titles, bureaucracy and army. Quite obviously different states live by completely different laws, what is acceptable in one state is unimaginable in another. Decision-making processes that are traditional in one state are unacceptable for another. According to your theory that culture (and here I mean religion, traditions, common language and ideology) is not important, how could you explain those differences?
When Roman and romanized population of the Empire began to dwindle (and there's no doubt that depopulation process was under way in late antiquity in those parts of the world), it was gradually replaced by the influx of not romanized barbarians. The empire could not romanize them unlike early empire which successfully romanized most of it's conquered population. Interesting question that I can't answer is why that happened, why Gauls, Iberian tribes and many others were gradually romanized and in time become true "citizens" of the empire, while 4-th, 5-th century invaders were not. In any case, the result was that vast areas of the empire were populated mostly by non-Romans, again not merely by ethnicity, but by language, traditions and religion and it's very hard to sustain a Roman Empire without Romans. The issue is very complex, but I hope the bottom line is clear.
What you write about the Byzantians is only partially true. Yes, they still called themselves Romans, but for the most part of its history Byzantian Empire had very little in common with the Roman Empire of the ancient era. In one of the books about the Byzantian Empire I've even read that it ceased to be "Roman" after Arab sieges of Constantinople. I don't know Byzantian history well enough to comment on this with some authority, but the author says that after Arab wars the empire had undergone deep changes that made anything Roman Byzantians still had no more than just a "heritage".
P.S. There are several great posts by cmacq, but after reading them I still couldn't find out what was the cause for the depopulation, which also in my opinion was one of the main reasons for the decline of the WRE.
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