I think that assimilation and integration are general a good things, but I think the most important things with a immigration are these things:
1: Do the immigrants cause problems or disruptions?
2: How major disruption or problems do they cause?
3: How much do these problems reproduce in the next generations?
If you have a immigrant group which doesn't disrupt things or cause problems, I honestly don't care whether they integrate or assimilate. If we have a group causing some problems but these problems doesn't reproduce next generation, I think you can live with them.
If Xahar is a good citizen and his children grows up be good citizens, he can decides to live in his parallel society for all I care. As long as he doesn't cause problem for other, why should I care about his life choices or religious views, and to be honest I would also prefer none of my relatives or children marry a Muslim, so I would be hypocrite for condemning him for sharing my views.
What you fail to recognize, and I mean this sincerely, is that starting from a basis where you immediately start questioning whether immigrants are "causing problems" is the very source of tension. If you start with the assumption that immigrants will assimilate and that they are mostly good because they are people, they will assimilate because this assumption undergirds some form of birthright citizenship, which has been shown to dramatically improve integration at all levels. There's no serious debate about this!
Europeans who refuse to learn lessons from the immigrant society when dealing with immigration are welcome to do so, just as Americans who decide to believe that guns are good and universal healthcare isn't important are welcome to do so. They will bear a cost for this foolishness but one value of our countries is that we are free to entertain foolish thoughts.