I believe it was Bill Jordan who stated that if you can't solve the problem with 6 rounds 6 more probably isn't going to help.
With due respect to Bill Jordan ... what if your problem is 7 bad guys?
It is rare but it HAS happened, and people with either "high capacity" magazines, extra "normal" capacity mags, or more than one gun have stood the best chance of surviving.
I don't have any experience in actual gunfights (Thank the Lord!!!!) but from what I've read that's come from experts and vets who have been there, done that, people who get shot don't magically all die instantly, sometimes they don't even know they've been shot until the smoke clears. I know of one WW2 incident of a soldier who was shot through his skull by a rifleman on a Pacific Island and didn't know he'd been hit at all, leastwise through the brain, until his CO reached over and removed his helmet to show him the bullet holes. In hole, out hole. Bisected his brain through the corpus collosum, right down the centerline, perfectly, which is why he survived. A one in one bazillion oddity sure, but it happened.
Handguns are notoriously underpowered to the point defense experts say that they're useful as tools to use to fight your way to a rifle.
I've never played wargames with airsoft but I've seen a few that are electric and use those plastic BBs....I would NOT want to get hit with one, considering what I've seen them do, and anyone playing wargames even with the ones I've seen best wear eye protection or they will be blinded if hit there.
But that is not what happens in real battles. I'm sure 6 rounds will be enough for the majority of situations that us civies are likely to encounter, and yes I am sure things can happen fast. But there's always those "black swan" events that will bite us in the butt.
Which is why I remain a bit cynical at cute little quotes from even experts like Bill Jordan...we have better more reliable guns than the old Colt sixgun. Having a 12 -15 round magazine in a Sig or Beretta today; that's a GOOD THING.