Steve has been noting that multicultural societies are not particularly vibrant, while monocultural ones often are. I am not entirely unsympathetic to this idea, but,

1) One could quibble with defining Athens as monocultural, seeing as how metics formed a substantial portion of the population. Men like Anaxagoas and Aristotle are not trivial. But, they were Hellenes.

2) What about Christianity? Hebrew theology + Greek philosophy + Roman organization = Christianity? Right? In short, that's what we have to thank multicultural empires for: world religions (i.e., Syrian Christianity + Arab polytheism = Islam, Indian Buddhism + Daoism = Chan Buddhism, the examples are legion).

Whether you think this is a good thing or bad, that's up to you....