It’s a fantasy because it’s extremely unlikely to happen, and if it did happen, the likelihood that you would have the ability and time needed to access your AR-15 makes it seem less and less likely that you would ever use an AR-15 for self defense which is why they rarely are.
If you see something happening from 100 yards out and have time to prepare and shoot from a far, then sure.
Also, the other side of the debate for that shooting was that someone had a weapon who could kill 26 people in a matter of seconds. Asking why we need those weapons after events like that isn’t unreasonable.
My view is that we probably don’t really need them, and would be better off without them, but it’s too late to change so we’ll live with it.