So, if the Red Tower is no longer functional, how is Mundus still here? Does Nirn not need the Red Tower to be able to exist without the Divines? Or has Nirn become so powerful as to be a god in of itself?
Because their are multiple towers keeping the strings binding Mundus together from breaking. Red Tower was a very important tower (Since it contained the Heart of Lorkhan), but it's downfall doesn't mean all things would come to an end, it only meant that a string snapped, making Mundus less stable, that's why some people believe the Nerevarine was just Azura's puppet and, in a way, Dagoth Ur was a good guy.
White-Gold, which was the Amulet of Kings/Dragonfires, was broken, leading to the events of Oblivion, but then the tower was reconstructed when Martin merged with the essence of Akatosh/Shezzar.
However, all the towers (excluding Red Tower) are just imitations of Ada-Mantia and the Wheel, or something like that.