Hello. In light of the recent of announcement of the Legend of Korra, I have a theory about why all of Aang's friends have died out including him.
Is it possible the Fire Nation somehow rose up but (but very quietly) during the time we missed and created a world of bending haters to fight against Aang and his friends before Ozai's inevitable death? Maybe there was some unknown agents of Ozai's fire benders which had escaped the retribution Aang delivered at the end of the last series. They may have caused an incident to occur in which Aang and his friends were attacked and killed by a group of people with advanced weaponary who had been given Firebending powers somehow.
For some reason I think this is very plausible. Opinions?