Deflationary does not imply an ever-increasing value. At some point, deflationary commodities will be so rare as to be useless. Lack of supply leads to a lack of velocity, which leads to a lack of liquidity. Once that happens, people will move to a competing product.

Imagine if soy beans gradually went extinct. In the short-term, the prices would spike as companies reliant on soy beans compete for the shrinking supply. But as prices go up, these companies either go bankrupt, or find alternative inputs to replace soy beans. Either way, the prices eventually hit zero before extinction occurs.

The once plentiful and valuable becomes rare and worthless.