I suppose if drugs were decriminalised, and users could get them from legal sources, the dealers deprived of the possibility of making money that way would try to find some alternative racket, as happened when Prohibition of alcohol ended in the States.

But I think they'd find it pretty hard to find anything as lucrative or simple, and the second-order criminality of addicts desperate to get the money to fund their habit would be cut out pretty effectively.

The trouble with the prohibitionist approach is that it just does not work, and creates enormous problems in itself. It would seem far wider to adopt a different approach, drawing on the experience of other, legal drugs. Limit use in public-places, as with smoking, outlaw any kind of advertising, license certain types of provider, provide help and support for people trying to break habits. And at the same time pull the carpet from under the dealers by depriving them of their market.