Whoever holds a powered-up White-Gold Tower holds symbolic and physical dominion over Tamriel, or at least a very significant part of it. Currently White-Gold is, for all symbolic purposes, offline. Akatosh may or may not be the new gem, but even if he were, that'd only mean that he holds White-Gold Tower. Now, Umbriel is an Oblivionic version of a Tower with Umbra as its gem. It's purpose is to bring a point of Oblivion to Nirn and merge Umbriel with White-Gold, merging the Point with Nirn. Suppose this happens. White-Gold's gem would become supplanted (if it even has one to begin with) by Umbriel's gem. Umbra. Umbra eats the souls of both those who wield it and those who meet the other end, turning practically everything it touches into just Umbra. What do you suppose would happen to the rest of Tamriel if Umbra was allowed to remain powering White-Gold? White-Gold becomes Umbra. Tamriel becomes Umbra. Aedroth becomes Umbra. Umbra becomes Umbra.
I never got the impression that Umbriel had any intention of
staying at White-Gold - but rather just using it to pop its bubble of Oblivion and move on. That said, I like the idea.
Umbra is a powerful Daedric spirit, who was trapped in a sword. He broke free, ate a piece of Clavicus Vile and now acts as the "gem" for the floating city of Umbriel.
Where does it say Umbra is daedric?