"...But, there is one they fear..." - Esbern
They obviously don't consider anyone else a threat.
What if your champion of cyrodiil did nothing but eat at the feed bag and use alteration spells?
How'd they become the Champ in the first place then?
They fear the Dragonborn because he can use their powers as well. Dragonshouts level the playing field, making the Dragonborn the only real thread to their plans. A master of blade could be able to kill a dragon here or there, he still wouldn't be a threat to the Aldiun's plans. The Dragonborn will be killing the bulk amount of dragons, but that doesn't mean we will be alone. I can see a master of destuction magic being able to go toe to toe with a dragon easily.