I see no real advantage of gil sinks, within the current systems. For all intent and purpose, gil is only useful to acquire optional items. Little to no character advancement is possible with gil and every advancement gil allows has alternatives that will not cost any gil. This is because FFXIV uses a system of specialized currencies to acquire specialized items, leaving few options for the general currency. This is by design.
With this in mind, there is nothing that needs to be done to players hoarding gil, precisely because gil is near pointless. A player cannot be rich if his wealth is worthless. The housing system is a weird semi-important feature of the game. You can do without it, but specific instances, for free companies, requires them. It is not optional enough to be on the gil currency, but it is too optional to create a specialized currency for housing. My understanding is that housing was not meant to be this important, but additions in Heavensward and Stormblood have made the system more useful, therefore important, which is likely an oversight from the development team.
The idea of creating more gil sinks has the purpose of giving more value to the gil, which is designed to be of little importance. It is a contradiction.
To address the elephant in the room, exclusive, high costs items purchased by gil only encourages gil buyers to throw real money at their computer's monitor. This is the reason we have a cash shop. So many players are ready to trade virtual goods and time for money, that SquareEnix is bypassing the middle man and taking in all the profits, instead of none. I do not like it, but this is the way it is. Never forget that gil sellers would not be in business if there were no buyers. Game companies would not be using cash shops if the majority of gamers were happy to be grinding those items, for no additional cost.
If you know that by making gil-exclusive items, you encourage gil buyers, there are two answers with a subscription-based game.
1. Let the gil buyers make gil selling profitable.
2. Have an official cash shop, in which all those items are sold for real money.