The rest were probably in cut worlds.That makes no Sense. Xehanort maybe gone, but The Darkness is still there.
They made a big deal of there are New Seven Hearts and we supposdley get 4 out of 7. So its like F*ck the rest?? How does that make sense from a Story-telling sense.
You introduce a New System and its there for shits & Giggles. Even when the Official KH website even announce that there are New Seven Hearts, I'm pretty confident its a thing for the Next Saga.