After some thought, I think I know why Alduin wasn't so world-eater-y in TESV. Back in the days of the Dragon War, he broke from his usual function to lead the dragons and conquer the world and/or Skyrim before he is sent to the future. We see in TESV that his efforts are much more clearly to continue that effort with the dragons than to destroy the world. Therefore I think Alduin's power is directly tied with the actions he commits to with intention, such that he is at his most powerful only when dedicating himself to eating the world, however that also means he doesn't get to enjoy all the finer things in life like world domination and all that.
If the Towers and Stones keep only Mundus secure, then I wonder what happens when it is gone and if other realms persist, especially if it really is just another realm of Oblivion specially claimed by the Aedra. For all we know, Oblivion as a whole and its subsequent planes have no need for such supporting structures, yet they persist in time and space quite fine. That being said, our understanding of Oblivion cosmology and metaphysics is pretty limited except in relation to other realms.
If the Towers and Stones keep only Mundus secure, then I wonder what happens when it is gone and if other realms persist, especially if it really is just another realm of Oblivion specially claimed by the Aedra. For all we know, Oblivion as a whole and its subsequent planes have no need for such supporting structures, yet they persist in time and space quite fine. That being said, our understanding of Oblivion cosmology and metaphysics is pretty limited except in relation to other realms.