what happens is that there are multiple levels of in game economy that exist. One is the npc vendor economy, then there is also the player economy. The first one is static unless they make actual changes to the game. the second one relies on the first as a small portion of it's overall effectiveness, but (hopefully) would rely far more on the value of tradeable goods in the game
in a real economy it's not just the money that holds value. every single item that can be purchased also has a fluctuating value based on supply and demand.
In ffxiv that would be crafted items, tradeable gear, materia and consumables. those items have value based on the demand for the item compared to the available supply. A great example would be explorer's gear. When they first added it to the game, things like tabards and moccasins had a much higher value because the demand was high and the supply was low.
As people started doing the required 17 minute speed runs in efforts to get relics the supply of the items caught up and surpassed the demand, now with a few exceptions their value is significantly lower.
but that isn't a one way street. a prime example being the atomos exploit they just recently tried to fix. It was possible for people to get a crazy amount of GC seals in a very short period of time by purchasing low seal cost vendable items and then selling them to npcs for gil.
You see it most with RMT because that is all they do. though in reality that is happening all the time. In a well balanced game you shouldn't be too aware of it though. the problem is that FFXIV right now is not even remotely well balanced. (or rather it's balanced around an incredibly low value for gil.)
I would hope that the lowering of the gil cap is an effort to make each gil more valuable, the fairest way do to that would be to eliminate a percentage of total gil in the game. every person loses 90% of their gil, the value AND distribution of wealth as a whole stays the same, the quantity of gil is lowered significantly. if they were to just remove all gil that anyone had over the cap gil would be worth more than it is now, but not by much and it would really only take the wealthiest people at the top and take their % of the total wealth down. people under the cap would actually gain a higher % of the wealth.
When considering vendors, gil is a static value, because their prices and their quantities will never change. When considering a player driven economy it changes all the time, both based on the current quantity of gil AND based on the current quantity of items available and items needed/wanted by the community.