I am pretty much convinced that Reagan's war on drugs was mostly about destroying south american economies and exterminating or enslaving non-whites, rather than public health and safety. Because of that, I don't believe de-criminalization will lead to criminalization.
Add to that the potential medicinal benefits that will be better-studied and available to more people and there's really nothing to lose. DUI is still illegal, as is showing up to work high. Children will of course not be allowed anywhere near drugs and what grown adults do in their free time is really none of anyone else's business.
If we started treating the real source of the problem -- mental health -- then there would be far less appeal for these types of people to turn to drugs to begin with.
1- Drugs are cool, and not really an issue, people take them mostly reasonably.
2- Hey, drugs can be addictive, that's when people stop being reasonable
3- Too many drug addicts, it is a real society problem
4- People demand strict laws, drugs are not cool anymore
5- Parents, who have seen the damage done by drugs support these drastic laws, and tell their kids how bad drugs are
6- Kids, who didn't experience a real drug epidemic, start questioning what their parents told them, and realize that taken reasonably, drugs are not that bad
7- These kids, now adults, seeing how harsh anti-drug laws do more harm than drugs, demand more tolerance
8- Drug-related legislation softens, back to 1