It's not necessarily true that the last plane crashed. In the purgatory world, they all died at various points. For all we know, Hurley and Ben could have been guarding the island for until the end of mankind - but they still died. There's no "now" as Jack's father said, all of them area dead some sooner some later. I…
But a giant rock slamming into the earth is something I understand. I've read evidence that it has happened before, killing all the dinosaurs, and I understand the science of how it could happen again. A better question (to pluck from the show) is if I handed you a small metal box with a button on it and I told you…