I think some of the criticism are flawed. I get that for some people this movie left a lot of questions and seemed to not be unrealistic. I think people are missing the human element of this.
1. "The can't restart the human population with just two," and you are right they can't, that's the point. Humanity was screwed and there is nothing you can do about it. They aren't going back to restart humanity they are going back to live there lives.
2. "What happened to earth, it was never explained?" They did say and show a few things, earth air isn't just unbreathable it's toxic and never the end it seems it was caused by humans a mistake. So what was the mistake an experiment gone wrong? I don't think It was necessary to explain this, it's the end of earth and there is many realistic ways it can actually happened. I understand if people dislike the movie because they didn't explain a plot that wouldn't have mattered in the end.
3. "Who was the little girl?" We'll spoilers ahead I guess. She didn't exist, or more accurately, she did but she was in space and he imagined her on the ground.
"So, What was the point of that?" He (even though he never showed it) cared for iris, and the little girl was a hallucination that would motivate him to actually save the real iris. During the movie I wanted the little girl saved and technically I got that, she was saved.
I don't think the questions many people are asking to be logical which makes me think a lot of the idea of what the movie was going for flew over many people heads and I might be one of the few people who actually understood and enjoyed the movie and it's characters.
Which is I'm not giving this movie 5 stars. A great movie should be able to show it's story without too much confusion.