Gresham's Law: Bad money drives out good. If cryptocurrency is deflating, it's becoming more valuable with time. The dollar is inflating - it's becoming less valuable with time. So given the choice, I would pay with dollars and hold crypto. That's just normal human behavior. But while that may make crypto a good investment, it makes it a terrible unit of exchange, as you say (or at least a unit of exchange that will only be used as a last resort).