Much as I like to criticize multiculturalism, it was invented for a reason, or some form of it anyway. I am not talking about the obviously absurd idea that different cultures with different values can coexist in a society that considers their values to be equal, but about the idea that different cultures and representatives thereof can in general coexist in more or less the same place.
The idea that ethnic and cultural groups can live together or side-by-side peacefully started to spread after the WWII, although obviously there were countless examples of them doing so, peacefully or otherwise, throughout the human history.
At that point there was a lot of call for it in the Western world, what with WWI and WWII. People looked around, learned from experience, and said "shit, we have killed about 70 million of each other already this century, and kicked god knows how many out of their homes, maybe we should take a break?". They thought about it for a second (having learned more than one thing from experience) and said "ok, first let's kick the Germans minorities out, and then let's take a break". About 15 million of ethnic Germans got kicked out from wherever they lived, and everyone took a break, or at least everyone who was not occupied by communists.
I think that another contributing factor was that by that time everyone noticed that the many millions of various Europeans who immigrated to the US in the late 19th and early 20th century have totally failed to bring the country down, and assimilated quite well instead.
We (obviously speaking of the people who are critical of multiculturalism here) laugh when multiculturalists use the wide availability of various ethnic cuisines as the most commonly quoted benefit of multiculturalism (it's a safe and value-free choice, unlike, say, honor killings), but there are a lot of real benefits to the more tolerant approach to the different cultures that the West developed after WWII. Think about the fact that a war between Germany and France seems to be a rather absurd idea to anyone born after WWII. Or about the fact that some borders that were really bloody for centuries can now only be noticed by the change of your GSM operator. Or about how easy it is for you now to go to study or work in some other Western country.
The current refugee convention was also written after WWII, and for the obvious reasons. So many Europeans became refugees during the war that people wanted to ensure that there would be somewhere to run, just in case, and they did not consider admitting refugees to be a particularly high price for this kind of security.
Mass Muslim immigration into some European countries started in 1960s. The current sort of multiculturalism - the one that encourages immigrant groups to retain their cultural indentities - arose in the 1970s. I am not sure whether there is a causal relation. Probably not. Probably the generation who grew up with the idea that cultures can coexist peacefully somehow decided that it meant that all cultures can coexist peacefully.
The modern multiculturalism is based on a rather unrealistic view of human nature, and fails to answer two important questions:
1. Peaceful coexistence takes two. What do you do if the other culture does not wish to coexist peacefully?
2. What do you do if the other culture's values are absolutely incompatible with your own?
The actual answers that I have received to these questions were 1) try harder, all you need is love, and 2) but of course somebody will see the light and some kind of compromise can be achieved. This goes on the same lower cognitive shelf as "if the people just stop being selfish we will achieve communism" (labeled "jos tädillä olisi munat, se olisi setä").
Multiculturalism has outlived its use and its welcome. The useful lessons have been learned with its help; the new useful lessons are being learned in spite of it sticking fingers in its ears and screaming "la-la-la!". Different cultures coexisting peacefully is something everyone has already noticed, often to the extent that they take it for granted. Different cultures failing to coexist peacefully is an elephant in the living room that multiculturalists want to avoid talking about.
For most of the last century the Western world has been learning a lesson: different cultures can live together. The lesson was true, well-learned and very expensive. I am afraid that now we are learning the second part: "...but not all of them". I am afraid it will be expensive too. I have some idea of what the third part of the lesson might be, but I am not comfortable with discussing it in public.