Well Kingdom Hearts will last longer than we'd expected. But Sora's story is always important. Kingdom Hearts is not Kingdom Hearts without Sora. Even if Sora is not playable, Sora has ties to the new warriors by the Keyblade and possibly other means. Its weird that Nomura said that he could not say if Riku or Sora would appear in this installment. Although its a new project and experince, he can't officially say that Sora is out of it.
But eventually when this new series end, I bet there will be one more game based on Sora's last adventure and it will wrap up the series. This new series just ties everything together, at the end Sora will have to finish the series off as main character because it started with him, so naturally it will have to end with him. Remember Nomura believed that even by the time this series is over, Sora's story will never be not important.