Honestly, I don't care for yours too. Arguing with you is like talking to a brick wall.
Cops showing up doesn't end the situation every time, either. About 50% of rampage shootings end in suicides. Not "suicide by cop", straight up "Fuck you, I'm out." suicides. A good chunk of those happen before the cops get there. An overlapping chunk happened as soon as the assailant met resistance. Armed civilians means that resistance happens much, much sooner.
In that video, Grandpa may or may not have even hit with a single shot (the video isn't very clear). But he sure as hell ended the situation, now didn't he?
I guess we'll never know if you would prefer to be witness to a 15 second gun battle, or huddled under some scenery for 6 minutes while you wait for the cops to show up or the gunman to find you.