If a Muslim family on avg has kids around early 20s and have about 3kids each vs avg European family starting in early 30s and having 1 kid on avg..
In 60 yrs - Muslim pop will have 8x while European population will be at 1.75x
If Muslims are at 5%, they will then turn to 25% of population vs 75% Germans after 60 years.
The population change is exponential.
A lot of things that change you won’t even notice until it’s too late.
That assumes that reproduction rate stays constant. Fact is: after the second generation, reproduction rate is only a tiny bit above the one of the "host country" (per http://www.spiegel.de/politik/deutschland/demografische-stud...).
The reason is that while first generation women usually stay at home and care for kids, second and following generation women go to universities and then into jobs which means delayed childbirth and less kids overall (because each kid means a drop in career). In addition, Germany has easy and cheap access to family planning - everything from pill and condoms over "plan b pill" to abortions - than typical "origin countries", further lowering the rate of unwanted pregnancies.
Altogether, the "Umvolkung" theory is utter bullshit spewed out especially by the Identitäre Bewegung neo nazis.