The question is not 'Is the number of people doing drugs affected by its legality' (though I think its quite likely there is a fairly large effect.

Its 'Are there people who would do drugs (and become addicted) if they were legal but will not currently because they are illegal'.

Or in real life, how many potential alcoholics will be better off with cannabis as their drug of choice? Clearly the lifetime outcomes for heavy drinking are worse than for heavy cannabis usage, and they seem to be strong substitutes for each other.

What the discussion here is missing is that it would be nearly impossible to abuse mushrooms, and that mushrooms seem more likely to get people out of other addictions than push them into one.