Plex works in EVE because CCP simultaneously implemented a bloodhound team who are one step away from rabid when it comes to dealing with RMT and botters. A Plex system will NOT work in FFXIV due to SE's lax ban wave system where they "get around to it eventually." Putting a purchasable time system in, without taking a heavy and active hand against gil sellers, would simply enable them to empty out the market of all game time for sale, further fueling them.

As much as a system like this would benefit the game, it CANNOT function with current GM enforcement methods.