Illusion of unit of account.
Juraj Karpis said in his talk at HCPP that we have this almost a language notion of unit of account. We all know what our currency (1$, 1€) means. We don't have to think about it. BTC has not achieved this yet. See the talk, it's amazing:
But this has a flipside.
Level 1: "EUR in 2010" and "EUR in 2025" are very different currencies. Looks the same, same pictures, same unicode character (€), very different purchasing power.
Level 2: No, you can't multiply the 2010 prices with CPI over the years. 1000€ in 2010 would be 1368€ today according to these statistics. But that's completely wrong and we know that. They CPI is not calculated very well.
Level 3: Even if we got the "purchasing power level" right, the relative prices have changed. In 2010 I could not even get some things (like click in an app and someone would deliver me youghurt to my doorstep 30 mins later). A laptop that cost 1000€ in 2010 is basically a free gift today, or less than 100€. The relative prices are changing as well.