I think the point is that what emerged from the ruins of the Roman Empire was helped, in part, by other civilizations.
I read a book called "
How the Irish Saved Civilization" that details monks and scribes copying and vast collections of manuscripts about Western civilization and hiding them away through the centuries after Rome's fall, until a new society could learn from them. Fascinating book.
Ancient civilizations weren't so interdependent. Whatever befell one was not likely to affect another in the same wave.
Empire ... country ... :wiggles hands:
What befell USSR was not something that simultaneously affected other countries.
What if it were something like a fertility disease, or loss of oil or global warming? Those could affect virtually all countries
unilaterally.