Yeah, I agree that such "predictions" are mostly doomsaying apocalypticism.
However, I'm still worried about social cohesion and civil peace. You don't need a majority of Muslim immigrants to be violent for that to happen. How many people do you need to wage civil unrest -- on both sides? Could 5000 or 10,000 on both sides willing to take up arms be enough to, say, set Paris on fire?
I'd say rape/sexual violence is a huge problem, too. When you cramp groups of people who are mostly male (ca. 70%, IIRC) and young (most between age 15-40) into cramped camps, without the opportunity to work, without much else to do to pass the time, and lots of factors causing frustration -- no matter the culture or origin of that group, there would always be problems. (I'm even sure a similar thing would happen if you exposed native Germans of a similar demographic to these conditions.)
Now add to that their origins in cultures that rely more heavily on traditional gender roles, and where suppression of sexual impulses is extreme and enforced via social control, and of course the flipside will be an explosion of these suppressed impulses and lack of restraint.
As far as I can see, the main problem is not "Islam" and its demands per se, but the extreme hypocrisy that arises in cultures where there are extremely strict behavioral codes enforced via social control. You see a different, albeit not entirely different phenomenon among Catholic priests who molest kids, or Bible Belt conservatives who are all the more unhinged when nobody's looking. A society that expects from young males to not even talk to women or look them in the eyes, where even shaking hands with a woman is considered "sexual", where young males are expected to fully suppress their natural impulses, is a sure recipe for hypocrisy and disaster.
But you're right when you say that the doomsayers often underestimate the power of our societies to assimilate even Muslim immigrants, IMO. While those who don't may pose huge problems allright, a large number of them is successfully seduced and assimilated thanks to the benefits and attractiveness of open, liberal Western culture.