It's not nearly as true of a shotgun outdoors. Yes a shotgun will penetrate walls but it will dissipate energy very quickly after the shot disperses. You won't be able to kill someone at 100 yards with a shotgun unless you're using slugs. You most certainly can do that with a .223.
What makes you think that buckshot, let's say the most popular one (00), will be unable to kill somebody at 100 yards?
And why would you only worry about "killing" an innocent bystander? Wounding them is still a very serious problem.
100 yards...when I picture "in the city" I don't picture, in my mind's eye, 100 yard empty spaces behind your attacker(s). I mean, if there's nothing and nobody for 100 yards are you even in a city?
I still seriously question this theory that an AR15 is a poor home defense gun in the city, but good in the country, because of range.