This is a valid question, and it's one I've worried about, too. However, I'm confident the developers have also thought about this.
They are basically the DM. They have a responsibility to find a solution. They will have a lot of data to help them be efficient in this regard. They can find areas that have not been explored. They can place bread crumbs in the world for people to find. They might even incentivize PCs to make their own dungeons. I picture the benevolent hand of a deity, guiding the fates of the people.
Content creation appears to be slated as an ongoing process for CoE. Instances have been a staple for most MMOs, but CoE is not typical. Non-Instance dungeons have drawbacks of complexity and quantity, but compelling benefits of uniqueness. Solutions are there, but SBS is probably playing the cards close to the vest here. They don't have to have everything done at launch. It can be an ongoing process they adjust as the world changes, just like a DM.
CoE will not be the same experience for everyone, guaranteed. If someone wants to experience the same dungeons as everyone else, and grind them over and over, then CoE is not the right game. If someone wants to sit by the fire and hear stories of some adventurer who found a relic in a dangerous cave, that is what CoE will provide.
And if you were the one to find the relic, your stories might compel others to join you on the next adventure.