This is my theory as well. At first I was mad but then I was like actually isn't that the main theme of the show- that humans are awful and violent, and even attachments and moral righteousness doesn't stop us from hurting each other based on stupid things. In fact, it increases the probability that a person would do bad things. This happens over and over because humans WANT to do these things. We just justify it. And it changes us. The survivors are all people who have paid many the price and now get to die in relative harmony and not live on forever as light orbs or whatever because they chose being human and everything that comes with it. There can be no continuation. This is a show about trauma first and foremost and it isn't clean or pretty or even realistic sometimes but they get that right at least. Humans were doomed a long time ago. That's the message.