While the taxes do take out gil, the problem is that every 8 rank 30 or 40 leves gives on average 100k, and if everyone is doing that every 2 days you have huge amounts of gil entering the economy. The taxes take some out but no where near enough to make up for the amount put in. And the problem with that is that prices will keep rising and rising and rising non stop. A good economy (at least for a game) needs to have approximately the same amount of gil in the game at all times. This game essentially has constant inflation. At this rate, 2 years from now, items would probably cost more than 10 times as much as they currently do. For current players this isn't a huge problem. But it makes it nearly impossible for new players to "catch up" and be able to afford anything or purchase "incentive" items.
However what will likely happen is SE will move towards a game not focused on gil but focused on untradeable items. Not sure if thats good or bad.