I do not think that Desmond is going to be dying before reuniting with Penny...but you never know. This show does not confine us to what we want or expect as an outcome for any character. As far as time travel being a true scientific possibility...it isn't at this point in time, that is true. Yet many great scientists have pondered it, and theorized about it at great length for some reason. There is nothing on this show that is really scientifically accurate at this point in time. Planes do not break up in mid air only to have most of the passengers survive with a few bruises. Plane crashes do not heal paralyisis or cure cancer...... Every person on an airplane flight would not have the random in depth connections to each other that we see here. Maybe by chance one or two might have some strange crossing of paths, but the direct connections that all of these people seem to have to one another ...that would be theoretically impossible. This is a fictional drama, not a reality show. The producers say the answers will lie in the realm of scientific possibility, not stone cold science as we know it at this point. There is a huge difference. Matrix style manipulation, time travel, dreams, flashbacks, eutopian societies, brainwashing, secret research gone wild...... none of it is very realistic, but it sure is a fun ride.