The main effect of terror is destabilisation, which is also what breeds terrorism. Islamic terrorism in Europe turns people against their muslim demographics, and most importantly promotes populism and right-wing politics in general. Those are all pretty auto-destructive things. Some normal reactions to terrorism is to alienate some demographics or to exact revenge on some countries, both of which just create more recruits for the terrorist. It makes sense if you don't take that universal-islamic-state narrative too seriously.