A few drugs that are essentially largely used as alternatives to safer drugs could possibly reasonably remain illegal.

The question to me books down to what addicts do if the drug remains illegal. Getting all drug users over onto a legal supply matters. Doesn't need to be easy - prescriptions etc. for the worst ones would be reasonable.

But getting them onto a legal, regulated supply from providers that can nudge them towards treatment or safer alternatives is even more important for the highest risk drugs.