Virtual economies generally mirror real economies. Real economies don't have "gil sinks" where money just "disappears" into thin air. The problem with XIV was that it gave out gil way too easily via leves, and so nothing seemed expensive. Gil sinks haven't changed that one bit.
Adding a 5% tax (with a 4k limit) isn't going to address the inflation issue when leves are still giving out gil by the tens of thousands, and there's other ways of quickly amassing gil. The problem isn't that money isn't being taken out of the system; the problem is new money is being printed faster than its being deprecated.
In one sense, yes, gil sinks are needed. But not arbitrarily. There should be a game-mechanic based reason for it beyond "Oh yeah, this saves you 15 seconds." Airships are a good example. Another example would be NPC vendors which sold stuff you couldn't get from players.
To quote another style based on the "gold sink" wiki page someone else posted, here's another unintrusive idea:
NPC prices could be raised/lowered depending on how much of an item gets sold in any window. Like a real economy. So if you start unloading a bunch of crap, you'll get less gil than if you sell it slowly. Extend that to other areas of the game, and suddenly you're not being given ridiculous surcharges; suddenly it's more like a real-world supply-demand economy, except NPC prices are the "gil sink" and it's totally invisible.Another improvement to active sinking is to couple it to a feedback control system. Such systems can be designed to maintain a set of prices or asset ratios, and if properly set up can add a great deal of price stability to a virtual economy; one example of this can be found in the MMO MUD Alter Aeon.[2] The feedback control system used in Alter Aeon works by tracking the total amount of money in the game in order to dynamically adjust drop rates and shop prices. Players with more than 1 million in currency are taxed for 2% of the money they own over that limit. This keeps the economy permanently stable. The peaks in the total amount of in-game currency do not vary more than 10% in a time period of 2 years.