I personally wouldn't play if not for the goal for other story games.
It takes too long to gain 1 energy.
If you want players to spend money on the game, try different strategy TL
I deleted after I reached island 3 for previous goal and then uninstalled.
But it's taking more energy to go on to the next level now that I'm thinking maybe I should give up those gems for completing this goal and just uninstall for good.
1 energy for 5-10min will be good.