Again your basing this all on good faith. Do you really think people who have lived there life's by doing this illegal, are just going to up and go by the books because of it being legal? Why pay more when they can get it from across the border or grow it themselves and completely cut out hte middle man and have larger profits.
Also like you said people do it in moderation just for the hope of not getting caught. Make it legal and you remove that getting caught aspect. Meaning the moderation level changes.
Just like you said, no different then someone drinking heavly and going to work hung over. But like alcohol there are addictions and there are those that can be mentally addicted to marijuana if it becomes legal and with more oppurtunity to use it.