I realize this, but when you go and compare it to Perrif and Tiber it all seems to pale in comparison, especially when someone comes along and says "Oh, you defeated Alduin? Well he may be back later to finish the job. A+ for buying time though." It was the first time in TES where I truly felt like I did a whole lotta nothin in the course of the MQ, which is why I wish they managed to tie in the Thalmor, the course of the civil war and the like into the MQ apart from the negotiations with the Greybeards. (Which was a part I liked.) I realize its all about being a obscure hero, but thus far it feels like being a Dragonborn has 0 relevance in Skyrim aside from Balgruuf and a few awestruck guards when you consider what his/her predecessors have done.
I think you're dismissing it as a much smaller event than it actually was.
Sure, Alduin may come back, but that's what he does. When the world is ready to be eaten, that's his job. The world wasn't ready to be eaten yet, so as was told by the prophecies, the only being that could prevent this from happening, Dovahkiin, manifested and defeated the world eater.
It's not like anyone else could have done this. Dovahkiin is the only one who can prevent Alduin from eating the world and starting time all over again.
One day, Alduin may return again to attempt to eat the world, but if the world isn't ready to be eaten, Dovahkiin will be there to defeat Alduin once more.