I forget where I picked it up (FR?) but I once read that saying as:
“Power abhors a vacuum”
And with Islam being a political movement as well as a religious movement the muslims are also focused on power.
If Christianity has no power in the West then the muslims will see an opportunity where it presents itself.
What about the 1st amendment prohibition against state establishments of religion (like Sharia law) - does that still have any power?
2nd amendment survey says.... Yes.