There are, at this exact moment in time, a finite number of user experience problems. It is an extremely large number of problems and could probably keep this site going on for the rest of our lifetimes, all technology, software and people remaining the same.
But that's not how it works. We end up with new technologies, new interaction patterns and even new conventions, social expectations and laws. There's new psychological and market research to base decisions on, new findings to critically evaluate and test.
When we have tactile touch screens suddenly there will be a burst of new situations for questions. The same is true for if 3D becomes common for interfaces, or virtual reality, or smart homes/ect. To assume there is a limited amount of questions assumes there is a limited amount of work to be done. There isn't.
If this site was shifted back in time 6 years do you think we would have 200 mobile questions? No, because mobile was almost a fevered dream, cell phones were awkward dumb devices and Blackberries and PDAs were only used by the tech elite or savvy businessmen. Now we're on the road to Smartphones being more popular than just "phones". There wasn't responsive-design or android or ios, and a tablet was just a Wacom; only viable for the design industry, not normal users. In 6 years the world of tech has shifted more than other industries have in decades, and with that comes more UX challenges.
See also Ninefinger's excellent post on a Meta Stack Overflow post about the same topic.
Yes, we've covered many of the big issues in current UX. Yes, we had that discussion on the Floppy disc icon. Yes, we talked about progress bars animating "backwards". But there's still a lot of ground to explore and much of that ground to cover doesn't exist yet.