Well, the way I read the OP, he's asking whether or not Dragonborns and Tieflings have traction compared to more "traditional" races like Elves, Dwarves, and Halfings; he further asks if whatever traction Dragonborn or Tiefling characters have is merely the result of mechanical benefit. For my money, I don't think the OP is asking about the history of these races, nor is the history of these races directly in relation to either question.
To elaborate, since we're discussing the amount of traction Dragonborn and Tiefling have currently, I don't think it matters much whether or not they were first published twenty years ago or last Tuesday. What does matter is whether or not they do have traction with the fandom now, even if they did first appear only last Tuesday. The way I see it, the length of time a concept has been in publication doesn't have any direct bearing upon whether or not the current fanbase thinks something is interesting or not.
Its worth pointing out that it's not clear whether Eladrin qualify as a "traditional race" or a "new race", but what is clear is that they appear to be quite widely embraced.