I take your point, but you didn't fully consider the implications of using incorrect (but cheap) materials in a proper gun. Barrel erosion, damage to the receiver, and gun explosions are all highly probable occurrences for using incorrect ammunition inside a modern, well-designed firearm.
This is exactly why using one that is cheap to produce en-mass is attractive; When there is no tangible benefit to the "safer" gun (Because it is not actually safer, because you cannot get the ammunition and must use improvised ammo), and the expensive gun will be damaged by use, then a gun that you can simply throw away or melt down and recycle (print again) makes a compelling case.
Disposable firearms are not new, and are actually a very old idea. It's where the derringer found its market niche, and yes-- it was NOTORIOUSLY unsafe.
Neonivek:
In some parts of rural america, it actually is. Shooting tin-cans and bottles is still very much a thing in flyoverland USA. Simply because discharging a weapon within a city limits is wreckless endangerment does not mean that this is the case in very low population, and unincorporated areas, where a proper earthen backstop is present. Shooting into a culvert or the equivalent is a very bad idea due to risk of ricochet, but not everyone is terribly bright, and may people DO INDEED go target shooting as a hobby.