Carl got shot in the head. He's still walking around.
Kirkman clearly doesn't have any qualms about sacrificing "realism" for the sake of the narrative. Not that I think people "need to die" for the story to be realistic. Once your survivors have made it past a certain point, you'd expect the attrition rate to drop pretty significantly.
There are only four survivors of Atlanta who are still around. If Kirkman is to kill one off every time he needs to "shake things up", that's pretty damn unsustainable. And I like Michonne, but she's nowhere near as well developed a character as some of the others. Abraham is "Rick's male lieutenant/resident badass who may or may not turn against him" take three, counting Shane. Maggie and Sophia are bit characters at most, and none of the new group have anywhere near enough development to fill the shoes of Atlanta survivors.
Yeah no I'm aware about Carl, but I've always felt that he in this case should be the only exception. Plus, there's always the notion that Kirkman's been building all of this up to make his death that much more unbelieveable (and then of course that would then lead into Rick's downfall/death).