If you think about it, you really didn't save the world, sure, we stopped Alduin, and decided the war between the Imperials and the Stormcloaks, but there are still dragons on the loose, and Gods know what else, and back to the subject of Alduin, only, you, the Jarl of Whiterun, the blades and the Graybeards are the only ones who know about him, it's not like the common folk know some giant black hell-dragon is ploting to overtake the world, they just know Dragons have been brought back to life. To me, that's why the ending seems like a hollow victory, but then again, what are you going to do, and another thing, why does DragonRend only work on Alduin?