Whether in Europe or the U.S., mass immigration is the fastest and largest migration of people in human history and it needs to be discussed. There are political ramifications, social and cultural consequences, as well as economic effects.
A nation has border integrity in order to maintain a nation-states primary virtue: it allows for culturally and politically like-minded people to exist together. That is why distinct nations have distinctly different political and social characters. If every nation becomes an egalitarian mixture of people then every nation will be essentially the same. What happens to those people that wish to live a different way?
What happens to those that wish to be libertarian freedom lovers?
What about those that love communism?
What about countries that simply adore ping-pong?
Where do those people go when every nation has become homogenized?
The world is better when people are free to move to a nation but also when that nation is free to refuse them entry. Sometimes preserving national character is a good thing, especially if you like that national character.
The problem with the multicultural egalitarians is that they disdain personal choice. They discount that perhaps the French LIKE being French for better or worse. Perhaps the homogenous Japanese would prefer to stay homogenous even with the economic consequences of an aging population.
It is perfectly reasonable for some countries to embrace multiculturalism while others do not. There needs to be an open discussion concerning the issues of mass migration and its consequences because there's no turning back the clock once the deed is done and the buyers remorse on this one might be fatal.