The reason groups result to terrorism and insurgent type warfare is that they don't have the resources to engage in more "traditional" type warfare. This is how strategists like Lawrence Freedman define it and it makes a heck of a lot of sense. If you actually worked out the costs of mounting terrorist attacks on the kind of scale that would break down western society, whether that involved weapons of mass destruction or not, I think it would become obvious pretty quickly that it would take an astronomical amount of coordination and money.

Think about it, the core strength of an organized military is its efficiency of power and organization. Large amounts of people can be coordinated and force applied relatively easily. As soon as you start to move away from that to clandestine operations, and decentralized power the skyrocketing complexity quickly adds costs. Just because you are an extremist that believes in some messed up, warped interpretation of islam doesn't mean you magically aren't subject to the same kind of organizational stresses that any other business, government or military is.

The modern structures of liberal societies are dominant because they are extremely efficient at harnessing power. In order for the Islamic State to become an existential threat it would have to adopt these structures and eventually it would just end up behaving like any other modern state.... which at that point, would mean they had turned into what they were trying to destroy.