Prices decrease under a stable money supply because the economy grows. Goods and services are being produced more efficiently and are therefore more plentiful relative to the same quantity of money and therefore cheaper. Growth in the value of money will simply reflect baseline growth in the value of the economy as a whole. There are still much higher returns available to investors who are willing to take greater risks--no external motivation is necessary. Monetary expansion doesn't alter this basic relationship, it just ruins the currency as a reliable store of value.