[adventured]

It certainly doesn't have to be a physical threat. People respond extremely negatively historically to forces that alter the culture they are used to or prefer.

If you take 30 million people from African Muslim majority nations and drop them into France in the next few decades, would the French culture remain unchanged? No, it would change dramatically. The people of France would react as you'd expect to the notion of that cultural change. The people of France - in my opinion - largely believe their culture works, that it is very valuable, that it is beneficial to their quality of life. Why would they want to see it change substantially? There will be something like a billion plus more people living in Africa and the Middle East in just the next 25 or 30 years, while the population of Europe contracts. Europeans have one of two choices to make in that time: heavily lock down immigration, or allow vast cultural change (and I'm sure it'll vary widely from nation to nation what occurs).