User blog comment:Gemini12/How long has the world had the 3 great powers/@comment-3569609-20120624000233/@comment-3986064-20120624084830

Or maybe the older Shichibukai just died and on Thriller Bark we have not seen any of their corpses, there's a possibility later on in the series, but otherwise, if they're dead, there is really no reason for anyone to mention a dead Shichi anywhere since it's pretty much unneeded in the series. So the concept doesn't necessarily need to be new, but only the ones from the current storyline were shown and mentioned.