One of 2 ways, I think...1 - John Goodman's character lied and she takes off her mask and breaths, sees children playing on a field and runs to them and screen fades
2 - sees apocalypse, keeps mask on and screen pulls back to see a city on fire in the distance and fades
It's either John Goodman's character lied or he did not, these movies are never really about the final 5 mins but the journey. The movie chose the latter with a bit more action thrown in, I can forgive it for that and actually didn't mind it.
If it ended as per your option 1, I would have been annoyed. As that would be akin to it was "all a dream" ending.
Her walking out the bunker and fading to black and keeping the viewer guess would have been better.
Her running out and fighting with the alien seemed very contrived.
Her running out and fighting with the alien seemed very contrived.