Making alcohol illegal has been tried and is generally thought to be a rather bad idea (except for Al Capone).
The proponent of the "reversed argument" may argue, that, even if we can't ban alcohol (a real shame in their opinion), we shouldn't damage the situation any further with another evil substance. Later we may find that the new substance becomes "unbannable" as alcohol.
Canada legalized marijuana & not much changed
See also https://reason.com/2008/01/12/but-whatever-you-do-dont-legal
Therefore it's reasonable hypothesis that by outlawing a less harmful substance, we're causing more harm right now than we'd cause by allowing it. We don't know that for sure until we try the experiment, but it seems like a rational thing to try.