I have only skimmed your post, but I have thought about this problem as well recently. Essentially, players have to be invested in a game so it's not just about making a profit- otherwise everybody just sells everything at first chance. When players desire items from the game without financial incentive (or at least it becomes a secondary thing) then it becomes (more) sustainable. Also, I feel the more complex an economy is (like SPL) the more of a buffer it becomes for value to stay in a game. In that way it might take years for players to exist the game or parts of it which is enough time for others to enter and bring more $ into the game