@CrinkledEvidence KH III isn't going to be the last game, as stated by Tetsuya Nomura it is going to conclude the Xehanort Saga (Dark Seeker Saga?), but after that there is still several more loose ends that would need to be tied up. Too many to finish all in one game. Not to mention, I'm almost positive I read that Nomura said himself that there was more to come. KH III is going I follow Sora, Donald, and Goofy as they search for the seven guardians of light, while King Mickey and Riku gather the rest of the Keyblade wielders. But after that there is still other things left to do, ie. the seven guardians of light clashing with the thirteen vessels of darkness.
So don't be upset, there is more to come!