Nature abhors a vacuum. Christianity has all but vanished from the western European landscape. Some kind of religion is bound to takes it place. Scary.
I love that concept though I usually think of it in political terms. You are right though, there is a huge vacuum in the soul of Europe.
A few years ago I dated a rather pretty girl from Finland and she told me she had one basic rule..."no religious talk, no religion."
Being Southern, I stopped seeing her shortly thereafter but I always wondered where that idea came from.
I think you are right about the vacuum. But I suggest the vacuum is filled with anger and the thing that anger defends against...depression.
I don't think Europe will fall for Islam. I think it will respond violently against Islam. Of course, I could be wrong.