It is a valid point, that should be taken into consideration in the discussions on the topic, but it doesn't validate the original argument.
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.
By saying "we shouldn't damage the situation further," you're assuming that the harm of allowing the substance is worse than the harm of prohibiting it. With alcohol, we experimented and found that the harm of prohibiting it was worse than the harm of allowing it.
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.
It kinda does, because the nature of the ban is not any different, and neither are the social consequences. Prohibition failed not because consuming alcohol is actually healthy, but because the social costs of maintaining a meaningful ban far exceeded any advantages from reduced consumption. The same, then, applies to any drug that is not any more harmful.
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.