I think we're all forgetting that getting back 80% of the gil is supposed to be compensation for having lost your plot, it was not intended to be a way to sell your plot back to the system.

Naturally we all wait for the system to demolish the plot, myself included, but I think that's why SE haven't (and probably won't) added a way to instantly sell your plot back/get that money immediately.

Housing is one of the few gil sinks in this game, and even that was made less effective the moment the caretaker starting giving back 80% of the money. It's my personal belief that the caretaker only holds items/money for 35 days simply to try and make that gil sink more effective again. If they held it indefinitely it wouldn't really be much of a gil sink (20% being a lot less than 100%). A lot of people also buy their housing items/don't craft themselves, so in that respect I believe that furniture could also be counted in the gil sink.