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I think that is still mainly a consideration of the acute effects which occurred within the Roman empire, though, and not so much the effect that it had on the periphery of Rome and beyond its borders.
I agree with your points, I think bad things would happen within a Rome-like collapse of the US. But I think the overall global impact would be primarily limited to North America and other countries which are bound more closely to American geopolitics. But at the same time, given that the collapse of colonial structures restores autonomy to subjugated citizens of colonized regions, we see positive social development in the absence of Rome as I imagine we would see in the absence of America.
But who is Britannia? The Britons, who still led several uprisings trying to oust the Roman invaders? Do we follow the Roman lead of stopping the borders of Britannia arbitrarily at Caledonia and Hibernia and declare the people of those lands as being without value because they had less tribute to extract? Or do we look only at the accounts of the handful of British tribal kings who were willing to appease the Romans in exchange for preferential treatment, enough to be more positively written about in their surviving history?
Beyond Britannia, was Rome great for Judea? Did the tribes of Germania enjoy being invaded every time some emperor wanted to improve their legacy and try to one-up their forebears? Did the remaining Gaulish tribes miss Rome after the fall, if only because they were the only ones left alive after Caesar’s conquest? And, perhaps most importantly, was life in Rome great for all of the people enslaved by it throughout its history?
I really do get what you are saying, but keep in mind that Rome was a great place to be a Roman—it wasn’t so great for everyone else. There was violence and strife during its fall, but so was there in its rise and later stagnation. It’s mainly just lucky for Rome that they were the best record keepers of their time, to have written so many one-sided perspectives about how great it was, which certainly gave later Europe a wonderful ideal to miss after it was gone. But the foundations of that empire were nevertheless built on a brutal cycle of conquest, exploitation, and enslavement.