The brass took away the giggle switch from the crayon eaters to save on their ammo bill. There’s a reason “marining” is a verb, after all.
But every gun is designed to kill people, all the way back to the musket. And your suggestion of an integral magazine doesn’t do much, even if you could somehow round up all the ARs with detachable mags and “fix” them. The M1 Garand and it’s stripper clips are a historic example, and the modern ejection port mag loaders the neutered California ARs have to use make it trivial to reload.
You want to tackle this issue? Safe storage laws, building a culture around free, government-provided training and safety, and harsher punishments for NDs are a place to start. That’s not even getting into the quagmire that is our terrible healthcare system, and law enforcement that on average can’t do their jobs and act on tips that would stop many of the recent big mass shootings.
They said “choosing,” which is the key word in their statement. Some people don’t have a choice like you said, but that’s really just a matter of the push/pull forces of migration at this point.