Birth rates ain't static, and have been shown to decline. Immigration numbers ain't static either, and a buildup of negativity towards immigration is yet to build up properly in the population, which then needs to become reflected in the politics.
Also, remember that the strength of adherence to the religion changes over time among the followers.
If you look at many of these "muslim horror scenarios", they are not based on conservative numbers and does not take change in the variables user over time into consideration. On top of this, they are often mixed with some fear based propaganda.
If anything, these horror scenarios work well to stir up some opposition towards immigration among normal people. 
As more people see the destruction caused by immigration, more opposition against immigration and toughness towards muslims already living in the countries will be seen.
in France islam strike back...
children born in france in 70/90 from north african parent come back to islam, and radical islam,
like my father say (55 years old) i see more and more young arab wearing the barb and jellaba, i didnt' see that 10 years ago.
you can see more and more burka etc etc.
you speak about "strenght adherence to a religion", an arab adhere to the death to islam, you cant joke even a milisecond about islam.
i had two arab friend at my gym ten years ago, normal people, partying, enjoying life, for their study the go to university, with stranger student, the two guy fall in radical islam in one year, the barb etc, even their way to speak changed.
one of them disapear one year, when he come back, he look ten years older, i always believe that he was in a muslim camp somewhere in afganistan...
