That's a good point. It's also often the case that the differences aren't as big as they seem to outsiders. One of the benefits of going abroad for a while is to be able to see your own country's politics from a distance, and the one your in's closer. It's easy to see that the Overton window is often quite narrow.

However, I don't know many places where mainstream politics would accept political violence as anything but extreme in almost every case. I'm not sure the fringe is growing, but to come round to my initial point, I'm worried that it will after the last WWII generation dies out.