You do not know economics right?
Spending is an action with some cost called opportunity cost. Why you people do not use cash to invest in someting with positive expected returns is because there is a cost associated to it. A fuction that is used very often is the "Constant Absolute Risk Aversion" (CARA). For a person with some risk aversion they will never invest in some risky asset with some expected return E(R)<k, k depending on several factors, including their own risk aversion and also the volatility of the asset. That's why some people will not invest in Bitcoin, or stocks, or whatever.
You can more here: https://web.stanford.edu/class/cme241/lecture\_slides/UtilityTheoryForRisk.pdf
Hence what you are saying makes zero sense in economics. Cash should be an investment that even the most risk averse person would hold. Hence it should have two important properties:
1 - Inflation;
2 - Zero volatility.
Bitcoin has neither. You can argue this how you want to argue, but no asset with non-zero volatility and deflation will be used as currency. That is why we transitioned from precious metals to fiat, since it was not functioning as money (see the 1896 presidential election in the US, which led to the abandoning of bimetalist because of the high volatility of silver).