Yeah, Mullet has it. Even if any Espers were left alive, they would have died because 'magic will disappear from this world'. Without the statues' power (which Kefka had taken, and then it was lost when he was defeated), magic could not exist in the world. Because Terra was half-Esper, she too would die... except that Maduin says that if her 'human side' is strongly attached to someone or something (the children), she could live on as a normal human. With her Esper half disappearing, it's logical that any full Esper would disappear as well.