@AlexMCS That's not really true man. If there is no point in being selfless then there is no point in being selfish. It just depends on the person, since some people gain happiness from putting others before themselves.
@PrincessCake There is always a point in being selfish: Personal gain.
I'm not saiyng selfless people won't exist at all, but you will find out that the amount of people willing to put others above oneself for the sake of selflessness is far, far lower than the those who would embrace self-indulgence. And that's in our world, where most believe in some sort of Karma. Imagine in one that has no such thing.
Every post-apocalypse work emphasizes that, because that's just how living beings work on a basic level - self-preservation trumps all.