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[SamRF]

I know some people who have literally all retainers with max amount of gil. Not counting their FC banks. If the purpose is to break the RMT market, you'd be doing exactly the opposite giving these people the chance to convert Gil in IRL money in a rightful way.
That been said, you also have the majority of people in this game who doesn't have gil at all and that would either make the converting gil into game-time impossible, because of the ratio, or bringing even more RMT to the game.
Also, making gil in FF is something that makes you spend a lot of time, sitting in MB for example, undercutting people and stuff. You wouldn't be playing the game to get more game time, so what's the point?

Well you wouldn't really earn real life money but you'll save money for not spending on subscription. Perhaps make some kind of system in which you can only buy 1 subscription every month on 1 char so the gil billionaires can't completely wreck the supply. You would kind of break the RMT market since all money goes to SE instead of a third party (since you can only buy/sell subscriptions).

edit: RMT likely wouldn't go aways since WoW seems to also still have RMT, it would perhaps at least decrease it.