@Blectorn even tho i dont like sort of magical patch influance in game mechanic but i like your idea. it can be justifide like a corruption of officials who stolen money and melt them into golden nuggets and sold on a blackmarket to tribe of golblins or theft in a budget by criminals like a bank robbery or something who also then sold this gold to other people who become zombie and skeletons. that good narrative.
but also in such system merchants and regular people will be collecting their golden coins in banks, cos coins must have stucks and waight. so they will try to keep their gold in a banks of different cities. so if city will degrade they loose % of their gold due to mechanic you suggested. but not 10% its too many but maybe 1% or 0.5% of all people bank deposits goten stolen by officials. this will make investors paniced and withdraw all they mony and put in a different citybank.
or maybe bigger city means a bigger bank so if city degrade then bank get smaller and randome slots are disapeared which can contain gold, coins or and other valuable goods.
anyway i like this idea. it has potential.