Well I think they are still alive off somewhere...perhaps just travelling in a remote region or in clouds or somewhere they can't be found. B/C if they were completely gone Aang would be the last airbending avatar (as who would train him in another life?).
Even if there is not a group of nomads somewhere, I think some nomads must have survived as some could have escaped, been babies/children (& thus probably not killed), or been off on their own away from the group (like Aang, but not frozen) & then could have had children. Thus the culture could have died from lack of leaders who hold the majority of the traditions--the monks, but some "nomads", the actual people with the ability to bend could have survived, but not know HOW to bend (as might lose skills over time & have no one to teach them). Then Aang could find them eventually & somehow teach them, so technically he would be the last AirBENDER as his (surviving)people would not know how to bend (as Aang did not know how to bend water, earth, & fire originally except in a trance). Thus if Aang taught them there would be somebody to teach a new airbending avatar, the next time in the cycle...
