Price is a measure of instantaneous supply vs instantaneous demand. Demand for a currency will always grow over time. There will be more people creating new types of valuable goods and services in the future. products and people that don't even exist today.

More available goods/services chasing the same amount of available coins means currency prices will change (not be stable). To preserve stable prices the amount of currency available to be traded must scale with the amount of goods/services seeking to be traded.

Occasionally then amount of goods and services available for trade will fall compared with the amount of currency available for trade. That will need to be compensated for as well.