We all do it.  Whether it's social circles, education or economic status, or beliefs, we tend to gravitate to those closest to us.  So why would this be considered problematic as regards ethno-cultural factors?  It happens.  We generally prefer our own.  Even babies prefer their own.

Does this mean that everyone does it every time and in every way?  No, but it's demonstrably the case on the whole.

If people who are Chinese prefer other Chinese, is that wrong?  If so, how and why?  Likewise people of African or European heritage?  Keep in mind, this also breaks down into various subgroups: people of northern European heritage tend to prefer one another to southern and in turn eastern Europeans.  It breaks down further: does anyone with sense (and marginally a student of history) not believe there are internal fissures even in these larger subgroups?  The French and English — always pals?  How about Italians and Greeks?  The Balkans, anyone?  Anyone?  It goes from there.  If you survey the world's largest continent, what is a key thing you might notice about the people on it as far as its ethno-cultural makeup?  They are tribal as can be (and not always in a positive way, any more than it was in the Balkans back in the 1990s).  They prefer their own.

Perhaps it was more apocryphal than completely factual, but it is said that the great author James Baldwin encountered this during his travels through Africa.  It was at a bar or restaurant that he was asked what tribe he was from.  He said he wasn't; he was American.  Upon hearing this, all present turned their backs on him and shunned him.  In their minds, he was an outcast.  He had no tribe.  He had no people.

Now, can people integrate successfully?  Yes, maybe, depending.  Without a doubt, the more similar the groups are, the higher the probability of such integration.  No guarantees, and it's not even just a case of closeness in skin pigmentation, language, or religion.  The Irish Catholics who came to North America had a great deal of difficulty integrating into a majority-English-speaking, Caucasian, generally "Christianized" society.  So does anyone seriously believe that the greater the disparity between ethno-cultural groups, the lower the difficulty of integration?  

Has anyone in the governments of the West (regardless their political name or ideology) or the great minds of economic think-tanks or academe as a whole truly contemplated this, or have they only pushed a narrative based on talking points and made out of whole cloth?  I'd say it's more than likely the latter.  The consequences will be detrimental, if not devastating, to such countries.