I believe what may keep it relevant is it being an alternative to leveling other classes. Now yes, it has it's own leveling system but they also said some of the exp will count towards your class/job outside of the Deep Dungeon as well. Depending on how much (or little) that exp is, it could very easily keep people going there as an alternative means of leveling other classes. That alone can keep it relevant as another alternative to FATE/Dungeon grinding.