There is an illusion that goods price should not change over a certain period (price level stability)

This is wrong, because the productivity is increasing all the time and creating more supply and less demand, every goods' value should drop constantly over time. And if you compare many different type of goods, the speed of their depreciation are all different

A currency that is increasing in value over time will reflect that truth more precisely. In another word, there will never be a price level stability if the productivity is improving. So the best currency should be the one with fixed supply, not the one that can keep the price level stable, which is impossible to achieve for all the goods at the same time