Exactly, cash has an opportunity cost which is the interest not earned by holding cash instead of investing it at the risk-free rate, and this is true regardles

Exactly, cash has an opportunity cost which is the interest not earned by holding cash instead of investing it at the risk-free rate, and this is true regardless of whether there is inflation or not. Therefore what makes cash a sub-optimal savings vehicle is the existence of risk-free investment opportunities with a positive rate of return. Nothing to do with inflation.