The only way to enforce a "compulsory rehab" is for the police to charge with possession and then for the court to sentence to a drug treatment program in lieu of prison. 

So you'd have to have the police chasing down every single addict, which would be astonishingly expensive and inefficient, and then to process every single addict through the courts, which would be more of the same. 

Unless you also advocate for a dramatic suspension of civil liberties, "compulsory rehab" is nothing but a pipe dream.