I don't remember the loss of the "revenge instinct" being a part of that book. The future humans had optimized for comfort/happiness so much that they evolved to be more and more docile and childlike. They lost their intelligence and evolved into stupid beasts again because they had no evolutionary pressure to become anything else. They didn't lose their self-preservatory instinct -- they just lost the only defensive tool humans have evolved that lets us compete/win against all other animals: our intelligence. Kind of like those birds in a New Zealand island that evolved to be too fat to fly, because the island had no predators. And then cats got accidentally introduced to the island.