You cannot, but you can increase money supply money supply more stably when following average GDP growth, and increase money supply more when velocity decreased- and atrophy the supply when it increases. And a currency is much more than just what people can spend at a store. It is what people keep their savings in, what companies pay their employees in, what banks lend.

This cannot be done with an unstable currency- you cannot have a debt that will either go up or down 20% in value in the same year. I do not think fiats are inherently more stable, but some fiats have proven to be somewhat more stable because of responsible central banking- its not a good idea to count on central banks being responsible for ever. But essentially all widely spread cryptos continue to have a significant amount held by speculators and therefore they cannot be stable currencies.