Up until now, that was a wise rule of thumb, when currencies were invariable inflated constantly.  That does not mean it is an absolute truth.  Indeed, why not invest in a currency with a fixed monetary base?  If it ends up being used, and the savings rate and money velocity remains the same, you will gain purchasing power equal to the increase in productivity increase and population growth.  Bitcoin is still experimental, but in the future it could end up becoming a standard and perhaps even the safest of all investments.


At the moment, volatility is way too high to be 'preserving your purchasing power'.  It is not only an investment, but a wildly speculative one (but in my mind a very asymmetric one in risk/return).