What purpose does it serve? They were looking for a justification to not even make a last ditch effort to save the kid, yet her answer was "I have seen worse"? It's not so much letting the kids die, but the precious seconds wasted to debate letting him die instead of doing something such as opening the door to let him through, or looking for something to barricade the door in case they were able to break it open, or to boost up to the top of the car and run over to the next car, open the top and bang on it to get the walkers to turn around to the new noises. The kid has a knife he used to kill himself surely he can dispatch a few walkers on his own to hold on a bit longer. Also very surprised Negan and Gabriel did not oppose Maggie's decision to let go. Negan didn't help Maggie, true, but he tried to help the kid in the last episode. He has always been one to protect kids so I would expect him to try it, what is Maggie going to do, shoot him in the back for trying to help? What about Gabriel, who in the back of his mind must be thinking about how he closed the church doors and windows to keep his congregation out, so now he is faced with the same situation, basically a chance to do that over and he just stood there and said nothing? That made no sense. In the end they spent all their bullets to clear the walkers anyway.