At first when I heard that Desmond had to make a sacrifice, I immediately thought that he may end up dying in order to stop the MiB but after watching this episode, I get the feeling that the sacrifice he is going to have to make is choosing between the two timelines. Really its all in his hands right now. He can tell the others whatever he wants and could continue to live in the FST and will fall in love with Penny all over again and start their life together but doom the others on the island in the OT. Or, Desmond will make the choice to go with the OT and will stop the MiB but in doing so perhaps something will happen where he will never be able to get back to Penny and Charlie again. Like every story, there is going to be a happy ending for some and not so happy endings for others. Charlie and Faraday had to die because that is the life that fate played out for them. While it is not a happy ending, it is the ending for them nonetheless. In the FST, Charlie and Faraday have both realized this and accepted it. But what will Desmond choose?