Purchasing power is a function of a whole ton of things, including supply chains. If goods require more inputs or are less efficient to produce that will increase their price. This in turn decreases the relative purchasing power of a dollar. This can happen due to all sorts of externalities, for instance a tax. Or it can go down due to efficiencies in manufacturing technology or biotech. Or, a massive global pandemic leading to supply chain disruptions can cause prices to go up. Or housing can become more expensive because of zoning rules.

The "supply of currency units" is a fundamentally inadequate measure to capture this. It is too simplistic. Nobody takes it seriously except for a small group of very vocal online crackpots because it is so obviously unfit for purpose. [1]

We re-defined it as our understanding grew. The way we update practically any model in the face of new evidence.

Japan single-handedly demolishes the Austrian model. Their M2 supply grew 3X from 1990 to present but inflation remained 0% measured over thirty two years. Prices did not change from 1990 to 2022. [2, 3]

[1] https://www.pragcap.com/understanding-why-austrian-economics...

[2] https://fred.stlouisfed.org/series/JPNCPIALLMINMEI

[3] https://tradingeconomics.com/japan/money-supply-m2