I have 2 concerns about the arcadia coin change
1. it will have the opposite effect on the economy than what you had hoped and as diamonds become scarcer people will want more diamonds for what they are selling potentially driving up the costs of gear and creating an even bigger divide between new players and end game players.
2. with the reduction in value from arcadia coins the mid game content will become less desirable leading to people not wanting to do tikal/kalin/IDK etc. which will essentially put a block on new players progressing into the end game.
I really hope I am wrong about both these points, and only time will tell what the outcome will be.
Maybe implementing or improving something to incentivise people to do the mid-game content maybe something like a title for running instances X amount of times or rewards for helping people through the quests.