If the USD loses on average 50% of its purchasing power every 18?
The essential point of your statement, that holding money in cash long-term is a way to lose out, is indeed correct. As for what to buy in order to maintain or even increase the purchasing power of your investment, well, that is a complicated question. People invest in real estate; gold; stocks; bonds issued by governments and/or by companies; and many more. There is no right answer, and most people investing for the long term would be well-advised to spread their investments over several different asset classes.