Does anyone here remember Enoch Powell?
Yes, he was branded a racist but saw what was coming and warned people. People should have listened. Instead, they just label people racist who warn of the threats to civilization and society.
I don't know why "multiculturalism" is brandished as something to achieve or aspire to. The extent most people want it is to experience other cultures on vacation and to have more choices at the food court.
Some cultures get along and are more sub-sets of a bigger culture (like Irish, Italian, French etc... are all European / Western). Others are very different but there is no conflict or animosity because there is respect between them and we can live together fine (many Asian / Eastern countries). Most multiculturalism still really mean ethnic enclaves or areas (just no outright violence).
Then there is Islam.
Islam doesn't want to assimilate and it doesn't really want to get along (except as a temporary tactic to further it's goals of domination). The mandate of Islam is to conquer and subjugate. It's baked into the pie, it's the lettering that runs through the stick of rock, whatever analogy you want to use. It wants to be dominant and in control and if you think it doesn't and want to actively fight those giving the warnings then you'll feel it's boot on your neck one day.