I think FST-Desmond needs to show the 815ers that whatever they did, worked. To us, it's whatever the OT 815ers are about to do to sink the island, kill the MIB and set things right.
So far, life is better for everyone.
Sun and Jin are alive, pregnant, happy. Jack has a son, is a surgeon, not a drunk or addict Desmond will find Penny Charlie is alive and will find claire. Hurley is still rich, and not cursed Sawyer will find justice and in the end, without killing Anthony Cooper. Locke will walk again after getting Surgery from Jack. Sayid is arrested, but at least he's alive. Kate will be proven innocent.