One other thing to note: I and my friends were unable to locate drugs 90% of the time we wanted to ,despite our best efforts many weekends. It's not ubiquitous access to any substance on every street corner as presented in the crux of your statement. The prohibition DID make it more difficult for us most of the time. The big metros are a different story, but my story is quite similar to many in 50-250K people small cities.
It would be irresponsible to not afford protection against mass proliferation of drugs upon legalization... would have to think it through very carefully.
Ultimately I agree with you though, I think unfettered access would actually highlight more distinctly the pitfalls of being involved daily with recreational drug use, leading to a more enlightened population. Much of the problem initially would be created by the transition and integration rather than the ultimate discipline. After the transition I would think everything will level off. Still have to be prepared nonetheless, and I would NOT tolerate contributing to a minor to ANY degree. Automatic one year in jail or worse.