The excellent blog ACOUP has just had a five-part series, "The Queen's Latin", in part describing mass cultural transmission from Rome to various ethnic groups in Europe. TLDR: it helps to have a culture that readily accommodates different religions and other regional variations.
Cultures are not fixed for all time; they change all the time.
So for many of these countries you're asking for a lobotomy.
We're also not great at scaling trust.
"We the people of Rojava in Northern Syria: Kurds, Arabs, Assyrians, Turkmens, Armenians, Chechens, Circassians, among them Muslims, Christians, Yazidis and various other creeds and sects, declare that the nationalist State has placed Kurdistan, Mesopotamia and Syria at the heart of the chaos that is afflicting the Middle East and has brought acute crises and suffering upon the people. ... The Democratic Federation of Northern Syria is founded on a geographical concept"
(https://www.scribd.com/document/441234886/Social-Contract-of...)
And yes, trust doesn't really scale well, which is why centralized governance in general is a big problem. But there are known techniques to decentralize it in ways that can still scale, e.g. council democracy.
Honestly that comment doesn't read "racist" to me. Maybe the disconnect is taking "culture" too literally, or using "culture" when one really means "people" (i.e. a combination of culture and ancestry/descent). Also even though an entire culture can technically be transmitted memetically, that's practically not the case except in cases of early childhood adoption. Immigrant populations may tend to take on certain cultural characteristics (e.g. economic and political ones) of the population they get socialized into, but those characteristics may not actually be the ones particular individual cares the most about preserving (e.g. culture in a narrower non-anthropological sense, like religion and and attachment to certain literature).
And even assuming a full memetic replication of a culture, I don't think the last member of a colonized people would be entirely happy that his people went extinct even if some of the colonizers fully took up his culture.