The crypto money supply is predetermined, but demand is not, often driven by events outside of the blockchain. Supply needs to actively balance demand if the price is to stay stable.
For a stable price you'd need a crypto that adjusts mining difficulty in response to changes in the amount of demand (or something like that? My understanding of crypto currencies is only surface level).
However, the USD-to-Crypto price is something outside of the blockchain, so as far as my understanding goes you can't really use that as an input because you can't get undisputable information on the price, because the entirety of undisputable information should exist in the blockchain.
I guess you could try to use transaction volume as an input but that seems like a bad metric because it's a) gameable and 2) does not necessarily correlate with external demand for the crypto currency.