My personal belief is that the primary culprit is the price of energy. Energy prices dropped exponentially for 100 years, and then since the early 70s it has been relatively constant, roughly tracking inflation.
When the money supply can be changed so dramatically, how do I know whether energy prices are stable because the comparative value of energy is stable or whether its because fiscal policy is being manipulated to keep the appearance of a stable price?
I'd like to reach for inflation numbers and inflation adjusted prices. That requires an extremely reliable inflation metric though, and when I go down the rabbit hole of how inflation is tracked and how that has changed over time I just don't trust it enough to lean on it for comparison.
"Hard money" based on a tangible asset has definite problems, but prices are much easier to track. Comparing energy prices to a gold backed currency seems much more reliable than comparing against a fist currency that has seems a huge growth in the money supply over the last 80ish years.