Selfishness, is the core theme of this movie IMHO.
Cooper(dad) for his family, Brand(dad) for the safety of his daughter, Brand(daughter) for wanting to be with her lover, Humans, trying to survive their impending doom, Cooper(Muuuuuurph!), for not wanting her dad to leave her to save humanity instead. Freakin' Matt-Martian-Damon for wanting to stay alive! (I hope you don't make it out of Mars, jerk! lol)
Humans are selfish, we have been, through generations and always will be!
And it would've continued on and on, in the movie's universe. But in the end, that one selfless act by daddy-Cooper: going into the black hole, and never hoping to return that saves 'em all; his family, friends and the entire world.
That transition from a selfish/dying world to better one, it all hinged on that one act of selflessness!
And that selflessness could never have been highlighted so starkly if it weren't for all those selfish acts going around.