Actually, my interpretation from what they said is that a new expansion would be possible, but it wouldn't actually be an "expansion" in the sense of what most people have come to expect.

They were pretty clear in that they can't create new graphic resources for the game. That means no new NPCs, no new maps, etc. However, they could conceivably do something that completely reuses existing zones, characters, monsters, etc.

This game is so large, I think they could actually do expansions like that and pull it off. At the same time, I wouldn't hold my breath on that.