Given that a shotgun must cycle through its magazine in order (although you could eject unfired rounds), the second assumption is that by the time subsequent rounds are fired, the situation has escalated.

E.g. If you bury a load of birdshot in a wall or someone's skin and they're still threatening you, you likely want to escalate (instead of repeating same). And from the counter-point, you probably don't want your initial choice to be killing someone vs not firing.

As Dick Cheney demonstrated, catching a shotgun shell of smaller pellets usually isn't lethal, as they individually don't have enough penetration to hit vital organs. Or, same, but in sheetrock.

On the other hand, as parent argued, it's adding complexity to an already stressful situation and does make a lot of assumptions about shooter and attacker intent.