There's a bunch of stuff mixed in there, from primitive idiocy to elements of geopolitical gamesmanship.
If you take the "grassroots" problem - the games of "tolerance" to immigrants flooding France - that was the foundation of the problem. About 15 years ago I flew through France, Paris, De Gaulle airport, and had time to walk around Paris. No offense - I did not see a single "emigrant" who showed any benefit to the country that gave him shelter and the benefits of civilization. They were all consumers, all were quite aggressive towards the local population, and did not give a damn about France and its laws. I was then talking to acquaintances who lived in Paris, and to my statement "soon they will be a huge mass, which already considers France as their land, and you MUST HAVE THEM, do you want to start pre-emptive measures to take", I was told - "well what do you say!? they are people, they suffered there, we are civilized people, they will adapt and become full citizens of France". I laughed back, they were offended
It turned out I was right...
I beg to disagree with you on the issue of "benefits from the immigrants"
I think to a large extent, France has benefited immensely from immigrant population, ranging from a young protective work force. Take a look at France national football team, it depicts to a large extent to which they've benefited from immigrants. However, there might be one two Judas within the immigrant population, but that doesn't make the problems squarely lie on them.