re: Existential. As in: 2. philosophy crucial in shaping individual destiny: in the context of existentialism, involved in or vital to the shaping of a person’s self-chosen mode of existence and moral stance with respect to the rest of the world.
Interesting how my world has changed since the 50s. Ages ago I could show up a few minutes for an airplane (or an event with the president) and be seated comfortably. My banker did not have to "know me" as well as my mother, nor did the phone company have to provide access on demand to law-enforcement (law enforcement would climb the pole and attach their own alligator clips). Civil society "worked" and created many services and charities that provided a helping hand that came with a sense of obligation of "someday you’ll be in my shoes helping others – so remember to pay it forward." I could carry my rifle to high school with me on the bus to practice with the shooting team. etc. etc.
This all fits within my definition of "self-chosen mode of existence" that the Left started attacking in the 60s and the jihadis caused both the Left and the Right to choose for me how I will live in this society. And though I can bring myself to tolerate the dictatorship of the majority (while fighting to change it) I am unwilling to accept violent outsiders making the victims pay in these settings. So yes, terrorism, (including violent crime, home invasion, all those things that make us slaves to our fears), etc. ARE existential threats to me, my family, my friends and my country.