Stress in life-threatening situation may cause, that simple tasks like reloading or removing safety is much harder, and tasks that involve more dexterity like mag changes is almost impossible to do by an unexperienced shooter.

That's why carrying a gun in different state that condition one is irresponsible and uneffective

During training you're not on adrenaline rush, you don't have sweated hands and 2s for life and death shot

When your equipment is against you

You always have to pick gun and holster that is reliable and suits you. You have to be 100% sure of your equipment, that will not fail you when seconds matter. If you find your gun tangled in a holster - change holster. Train at least 15 minutes a day. Care about every repetition.

Big hole in the back

Remember, that delay in perception also applies to end of an attack. Police recorded a lot of cases when shots were taken from close-ups to suspect's back because police officers couldn't tell fast enough if assault ended.

Adrenaline is present also in attacker system. Polish police has in its records suspect, that was able to shoot at officer for 30 seconds after shot in hearth.

Summary

We can't tell one way or another if a 21ft rule is right or wrong. A lot depends on the situation that you're at, from the attacker and his physical shape, but **you have control over your preparation and equipment **. Remember:

Train always with timer and take a good care of every repetition like you would fight especially, when you concealed carry a gun for self-defense.