Been doing a lot of thinking on that ending, and how it reinforces that the movie is the story of how Michelle changes. Specifically how she goes from running from her problems to confronting her problems.
She begins the film unable and unwilling to face her problems. Something happened with her husband, and she packs up and leaves. She is literally running away. But the movie makes a point that this doesn't solve her problems. They still follow her. Ben calls, she answers, but she isn't even willing to talk. In fact, the problem is still so present for her despite her attempt to escape it that it completely distracts her from what's really going on. It distracts her from the radio giving early warning signs/clues to a massive event. It distracts her from the truck about to crash into her. She hasn't really escaped her problem at all. In this way she begins the film trapped, long before she ever wakes up in the bunker.
Once she does wake up in the bunker she's forced to confront her problems head on. She has no other choice if she's going to survive. The circumstance necessities that she does so. One of the first plans she formulates and attempts is to again run away, but this quickly proves to not be an option. So she must confront her problems directly. It forces her to adapt, to become increasingly aware of her surroundings, her position, her situation. It takes time, patience, and effort, but she works on it. She thereby grows as a person due to her experience in the bunker. It is training her how to manage and deal with her problems.
When things inevitably go crazy, she is able to spring into action and finally get out of the bunker. But getting out of the bunker isn't the end. That is merely escaping, running away. As I said, she starts the movie trapped. Her problems don't begin and end with Howard and the bunker, the problems are really external from that. Literally. This is why the film can't end here.
Enter the aliens, the big, giant, loud metaphor for all of this. Everything that has happened in the bunker has prepared her to face this external problem. She eventually winds up in the pickup truck, a counterpoint her car ride at the beginning of the film. Whereas then the problem was manifesting from inside the vehicle with the phone as surrogate, and thereby manifesting from her, now the threat is external, and the objects inside the car are her tools to deal with it. She is able to deal with it.
Far from the character at the beginning of the film who was so distracted by her problem she was blinded, she is now hyper aware. She is able to see and process what is going on, and how to deal with it head on. She processes the air is clean, that sounds get the attention of the aliens, that the gas is flammable. All these small moments and details that she's taking in and can use. So when she sees the bottle of booze she grabbed at the beginning of the film inside the vehicle, she is able to process the information and put it to use, obliterating the giant external problem threatening her. She's pulling the strength and tools to fight from within.
This is the climax of what she learned about herself and how she changed, but the final scene solidifies it. Her taking down the alien ship was again her being forced to confront her problems, so it's incredibly important that at the end she makes a choice to continue to confront her problems. She is making a choice to fight, and more than that to help others fight as well.
She starts the film running and ends the film making the choice to confront, and we get that character arc in an incredibly competent, tight, and confident film that wastes nothing and deftly navigates genre. EDIT: (Just realized the first track on the soundtrack is titled "Michelle" and the last track before the end credits track is "The New Michelle.")