So you think there are a lot of people who currently avoid drugs because they're illegal, but who would under legalization not avoid using to the point of disorderliness and unproductivity? Why should anyone believe this? Are you such a person?
I do know people who carefully avoid breaking the law.
These people would not try drugs: they are illegal.
If the drugs were legal, these people might be willing to “give it a taste”. Why not? It’s perfectly legal.
And some people who try drugs find that they they like them. A lot. It can become a problem.
I don’t mean to overstate the case, but the number of people who would find their lives messed up by access to legal drugs is greater than zero.