6 months in prison on first offense would get rid of the drug problem. Users.
Would congress go that far?
The illicit drug trade is the major reason why will still have cash and are not forced into digital currency.
If the government wanted to eliminate cash and force everyone into digital currency they would have to legalize drugs.
Then, large corporations under strict federal control would take over the business. All the small players using cash would be driven out via competition and regulation.
That said, prohibition will not eliminate or significantly reduce drug use. We only need to look at the failed experiment with alcohol prohibition.