Well. I wondered when posting the final version of the game if I had made the goal clear enough in the intro, but now I fear I won't like the realistic answer °^°
The end goal is actually to solve each of the four different problems in each world ; Energy and Etherial skills are a means to achieve the four victory conditions. (three because Oceanic world's events could not make it in on time)
All currencies are only used for event, indeed ; Energy is earned simply by exploring.
Although now that you bring it up, I really like the idea of converting unused physical currency after exhaustion: that would work well to encourage the player to spend more endurance for better rewards, at the cost of possibly less progress for the current incarnation. At first, endurance is really precious since the player doesn't know anything on the map, but the more you learn, the more endurance you can spend in events without worry, creating a natural exploring first into skill later transition. Thanks for the idea!