EDIT: To clarify, in general I also agree with the point you seem to be making that guns are dangerous and trying to make them seem less so by loading so-called “less lethal” ammunition makes it easier for the user to forget that. But if somebody wants birdshot, they should at least just load the whole tube with birdshot and accept that their load has different ballistic properties than buckshot.

As per my comment above, how I load my shotgun is mostly based around my willingness to inflict harm on somebody who has broken into my home. If I’m trying to limit impact, bird shot is lovely. If not, just load the whole tube with buck shot or slug or similar.

But trying to pre-plan an escalation of force via the shotgun tube ordering is a fools errand.

Police have an escalation of force via visibly different tools. A suspect can see the difference between a stick and a taser and a gun. And a cop can choose to draw any of these in any order. Stacking the tube locks you into a specific sequence that you have to remember and that your assailant can’t know.

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.