A no go zone is a very subjective idea anyway. What is it. Is it a place where you might get taunted or insulted as you walk by; or is it a place where you might be beaten or stabbed? It depends on who you ask.
But I would argue that your points work both ways. Islamic terrorism is definitely over-hyped in some places, trying to sell the impression that all or most Muslims are just straining at the leash to start blowing people up. But it is definitely and very systematically being played down in political circles and in the main-stream media.
Terrorism is not necessarily the thing that many people are afraid of anyway. Many are afraid of assaults and more minor abused, rising petty crimes rather than bloody atrocities. There is no way to claim that these have not multiplied frighteningly in areas of European cities where large numbers of Merkel’s guests have settled. This is a legitimate fear for people. It is also a legitimate fear that their streets, their culture and their way of life is being changed beyond recognition by a social engineering project that none of them were asked to vote on.
The part of your post that jumped out for me because I thought it was so unlike you, was this —
“They obviously aren't; but you could be forgiven for thinking each extremist faction doesn't really mind so much to risk reinforcing their enemies; as long as they themselves get correspondingly stronger against the overwhelming majority of peaceful, morally reasonable people.”
It is a comment worthy of a Guardian columnist I am afraid to say, because it assumes that anyone who doesn’t share your point of view is an extremist and those who do are morally reasonable people.
I do not think it is morally unreasonable for an Englishman or a Frenchman to want to protect his culture and his way of life. It also does not require that Englishman or Frenchman to hate anyone. But in many cases those “extremist” groups are made up mostly of people who are concerned about the changes that are being enforced upon their neighbourhoods against their will by people who never asked their permission.
I do not hate Muslims and I do not think most of them are terrorists or extremists; I also do not have a problem with immigration provided it is controlled and allowed in a small trickle rather than a flood. But I do not want 10,000 Muslims being landed on my home-town because I like it the way it is thanks; and I am no extremist. Nor do I think I am “morally unreasonable.”