Nah, the correct way to create a "buy currency" in a game is actually to remove all currency-as-a-reward from the game in the first place. That is why EVE's PLEX still works, where as Archeage 's APEX is a total failure. So the only way any money is introduced into the game is from people who bought it legitimately in the first place, and the only way it's destroyed is through market-linked sinks in the game. Hence in order to sell on the market, you need to buy money in the first place, or sell other things. There needs to be an equal amount of game money destruction for all that is created. So the way to do that?
Put a cost on failure.
Every crafted item, needs a cost to produce, and a cost to sell.
Every item needs a cost to repair, and a risk of destruction if repaired by a NPC, and the more skillful a NPC or player is, the higher the cost to repair (see desynth.)
Every item upgrade, needs a cost attached to it, failure needs a cost attached to it (kinda exists with Materia, but not really.)
Every dungeon, needs a cost to enter (here's a thing I'm glad I've not seen outside of Mabinogi,) and a cost of failure to complete.
Every time you die, needs a cost to be revived "here", and a cost to be revived "in the safe place"
Teleport, fast travel, etc needs a cost based on the linear distance. Heck, just remove teleports from the game entirely and do a point-to-point airship pickup and drop. No airship? Too bad, call a Airship taxi.
FFXIV has very little of this. Could you imagine the outrage if you had to buy a dungeon pass that lasts only one hour, and nobody queues for that dungeon? Happens all the time in Mabinogi. What if you entered the dungeon and due to lag, got immediately killed? Well better pay for that instant-revival from the cash shop... you did buy that before hand right?
Like the only real things I see here that FFXIV could attach costs to without making the players revolt would be costs to abandon the dungeon. eg "Cost to Vote Abandon: 100,000gil", insufficient funds? you can't hit yes. Costs to vote kick cost 100K. If the player d/c's and is kicked, they are given a 1 hour penalty unless they pay 100K to remove it. Costs relative to dungeon difficulty.
Like there just aren't enough ways to sink gold in this game without chasing casual players away. Everything is pointed towards easy and convenient. When was the last time you actually repaired your own gear? When was the last time you applied your own materia? Do you even apply any materia? Materia was one way to sink money, but then the rewards from the roulettes made that sink go away.
Unless FFXIV suddenly introduces airship-to-airship PvP battles where you have to build the ship yourself, and destruction of the ship is permanent, I just don't see any way to drain money out of the game fast enough.