I say this is an addiction because multiculturalism tends to have a noticeable progression in society, and I think in a psychological sense too, which mirrors that of an addiction. But this is quite subtle and difficult to put into words.
An addiction is something that starts small, where you have control over it for the most part and it doesn't affect other areas of your life too much, but gradually enlarges itself and you lose more and more control as it takes over more and more areas of your life. Eventually the addiction actually changes you at the level of personality and biology, and you become unable to value outside of the space of the addiction itself. You learn to see all else in life through the eyes of the addiction and everything that it implies. You learn to love the addiction even as you hate it. This is what makes addictions so difficult to break, because even as you want to break the addiction you also want to continue.
A little multiculturalism is normal and inevitable. But over time multiculturalism can creep larger and larger, and I do not just mean in the sense of actual people coming into your society from outside of that society, I also mean the attitude that comes with thinking that multiculturalism is a supreme value. Leftists/liberals see multicultural diversity in their society and literally only see this as good, they are incapable of seeing it as bad in any way. Why is this? I believe it is because the addiction of multiculturalism has entered later stages in them, and they are now seeing the world through the lenses of the addiction.
They are still riding high in the pleasure stage, even as the addiction is taking them over it has yet to exert more control over other areas of their lives, and they are still able to indulge the addiction without too much "withdrawal". But eventually these addicts will start to experience some of the negatives associated with their drug of choice, and will be forced to allow competing values into their value-sphere; but they will not do this, most of them will double down on the drug and continue to press forward in what amounts to basically a religious faith, the faith that the drug user has just at the point where the drug binges are starting to turn bad but he does not yet want to admit it.
Has there ever been a multicultural society in the history of the world? I don't think so, I cannot think of any examples. America has many different cultures residing within it, but America has always remained a monocultural society, where these other cultures represent micro-cultures that must to small or large degree conform to the main American culture or simply be denied access to cultural power on a wider scale. The existence of so many micro-cultures is possible only because of the existence of the monoculture that acts as stable substance in which the smaller "multicultural" elements can stably and rationally exist. Without a dominant monoculture, social cohesion and cultural functioning would quickly degrade because there would be no common basis on which these different groups and value-systems could interact with one another in deep, meaningful ways consistent over time.