We have numerous great examples of total societal collapse and eradication. Starvation to the point of extinction can do it on islands with no resources, disease can do it in previously isolated societies with no defense against it, invading armies can do it, and mass immigration and cultural conversion can do it. None of those is adaquate or available to a bunch of redneck bomb-chuckers. Not if they are trying to erradicate a society that spans countries, continents, languages, and armies.
Modern society simply does not face an existential threat from terrorists. They can do a lot of damage sure, but they will not erradicate modern society. If you want to suggest otherwise you are going to have to do better than "a few hundred thousand Americans die, the American government crumbles under the disaster relieve effort, then people start eating each other." I know you guys are really digging that fantasy, but you are delusional if you really think that would pose a threat to the modern global society.