SPOILERS:
I've never written a review before, but I think something many people here are forgetting is that all of these movies explore grief, and the way people cope with it. Yes, the action is amazing, but it's important for us to draw our own conclusions regarding the storytelling.
I'll admit, I was not very attached to the character in the beginning of the movie, and did not see the point of following someone with a terminal illness. However, after watching for a bit, I realized she didn't want to connect with anyone new because she was depressed, and wanted to die alone. That is why she did not try to physically ensure those two kids reached safety. They would have ended up taking care of her the way Aaron did. Additionally, she wanted pizza because her goal was to feel close to her Dad before death. When we're sick, who do we long for?? Our parents. It seems to me her Dad was all she had (which is why she's so attached to her cat). This is why Aaron takes her to see her Dad's piano after they realize the pizza place had burned down. It wasn't about the pizza, it was about her processing her Dad's death now that she is losing her ability to live in the same capacity. Even after the trauma she endured before and during the apocalypse, she was finally able to smile one last time before she died. To me, that means a lot.
No movie is perfect, but I think this one is just as good as the other two, it's just not as action-packed as people thought it would be.