The fact that money sitting in a checking account depreciates due to inflation is a terrible argument for cryptocurrency. No one actually does that with any significant amount of capital.
https://www.ons.gov.uk/economy/inflationandpriceindices/bull...
Diversifying with a deflationary asset like Bitcoin is a wise strategy, one that is likely responsible for its $160B market cap.
Cash (at least major currencies) is one asset class that is relatively safe but tends to depreciate due to inflation, especially due to recent monetary expansion by central banks. Cash can also be easily seized, and this is a cause of concern for some.
Contrasted with other asset classes that have yield, having all your net worth in cash would not be something you want to do.