Until it is. Its the things we don't concern ourselves with that often bite us hard on the butt.
While tru-ish sounding, that doesn't mean anything.
We all have plenty of things to concern ourselves with which are serious matters we should deal with or plan for. Things that can really harm us. Things that are likely, or at least in the realm of the statistically foreseeable. And things which we could react to in an effective way, so preparing for them is not a waste of precious mental and physical effort. (I.e.: not just so much "wheel spinning.")
As others have pointed out, if some jihadist plants a bomb or detonates a suicide vest there isn't anything my 8+1 rounds of .45ACP is going to do to help me. Best not to put much thought into that. (Aside from usual situational awareness and "don't BE there" habits.)
If 10-20 jihadists light up the local food court with bubba'd SKSs and AK clones -- again, I don't even play Steven Segal on TV, my pistol isn't going to be much of a defense. Carrying a full-size instead of a compact ... ok, sure. Big lotta good that's going to do.
One lonely martyr with a rifle? Well, what's the difference between him and Jimmy the disaffected goth teen off his psychotropics? We've been talking about those sorts of (astoundingly remote, but still far more likely) threats for decades. STILL not worth much worry. A gang shoot-out or basic hold up is still way, WAY more of a realistic concern.
And getting mugged in the parking lot is orders of magnitude more likely than that, even.
COULD it happen? Sure. WILL it happen? Maybe. Will it happen TO YOU? Uh, no. A statistician would put that up there with being hit by a meteorite. And, likewise with that chunk of space debris, if it DID happen, there wouldn't be anything you could do about it.
So don't spin your wheels. Improve your life and your safety in other, more pragmatic ways. If you could make changes to your life and habits, great! Do so in ways that would actually make a measurable, real difference. Don't go buy an AR "pistol" so you can tote more firepower.
Make a weekly or monthly habit of realistic practice with the gun you actually do carry. Or, heck, lose 20 lbs, stop smoking, stop texting while driving, pay off your debts, or any number of other things you could do with the time and money and mental effort you'd put into these new worries about Johnny Jihad. Stuff that matters.