Its the cuckoo problem. The differential in reproduction only works as long as the host nest's parents provide support. We in the West (Europe and US) do it to the underclasses through welfare.
Historically, people that were poor had lots of children so some of them would survive, because of the high mortality rate. Now, modern medicine and welfare gives a higher survival rate... so the underclass (or in this case, Muslim) cuckoo can outreproduce the host.
It is ultimately self limiting when the host is overwhelmed and stops providing food and medicine, but as the host, I'd rather find some other solution than the long term limit cycle which results in brutish poverty for all.
Why are Europeans, and for that matter, US intellectual and political "elites" too dumb to see this?
Because URPEANS(mostly) and Americans(generally) are socialists.. If they had any sense they wouldnt be socialists..
In Victorian England, this got reversed to a small degree. Many of the poor got jobs as servants to the wealthy and the upper middle, where their household work enabled the affluent to have more kids and have them cared for, while the servants were discouraged from having kids of their own.
I wonder if this contributed to the increasing prosperity of that era