Significance of 21 feet
Not sure what instructors are teaching that it's hard to justify a shoot outside of 21 feet.
The significance of 21 feet, as taught to me by USMC and USN instructors, is that the average shooter will not be able to draw, aim, and fire before a knife wielding attacker can close 21 feet. IE 21 feet is the MINIMUM distance to begin engaging a threat.
Point taken.
But you cannot disregard the fact that deployed military personnel have different rules of engagement than U.S. civilians on the street.
Let's say that you're on a public street and you see a guy about 30 feet away from you, who happens to be carrying a baseball bat, and he's walking your direction, closing the distance at a steady pace.
No threats whatsoever have been made toward you, but he is walking toward you.
He might be a guy who just came from a ball game.
He might be a guy who just bought a baseball bat for his kid.
Or he might be a guy who plans to crush your skull once he gets closer.
But until he makes a threatening remark or demonstrates obviously threatening behavior....
You don't have any LEGAL right to demand that he stop or change directions or drop the bat.
You don't have any LEGAL right to prevent him from walking right up to you.
You don't have any LEGAL right to even make him respond to any remarks you might make.
And you will have a very hard time explaining to a jury that you shot a guy simply because he was walking your direction, on a public street, while carrying a baseball bat.
IN OTHER WORDS.....
In the U.S.A. it is virtually impossible to prevent a street thug from closing the distance toward you unless he is stupid enough to threaten you at a distance.
And that is extremely rare.
Street thugs and muggers rely upon surprise and the ability to get close to their victims and they're usually smart enough to not tip their hand until the last second.
There's nothing wrong with practicing distance shooting.
But you should also practice shooting at arm's length distance, because THAT'S where you most likely to be attacked and have a justified defense.