First, force ghosts are like Buddha to me;putting off enlightenment to guide others to the same. You can't be afraid of death to be one, and all Sith are afraid of death in some capacity. A Sith is much more likely to keep their body alive unnaturally for thousands of years than be able to be a force ghost. And on the main matter, I'm certain they'll use the term Sith again, but I highly doubt they'll mention the prophecy. Sith just means dark side user to most movie-goers, and since for the most part the jedi are ignorant of Sith ways, it's unlikely greater knowledge would reach Luke before episode 7. Due to this, if he calls a new dark side user a Sith, it'd make sense, because to him that's all they are. The prophecy did destroy the Sith in my opinion, but not evil itself, and new people will be tempted to the dark side. They may even attempt to follow old Sith traditions, but since all Sith at the time were destroyed, I don't think this invalidates the prophecy.