IMHO if a limited number of mostly well-understood drugs (from different types) would be legalized, then it would seriously undercut the demand for any of the new substances.
I don't mean to detract from the point you were trying to make, but a lot of the novel psychoactive substances that have appeared in recent times (e.g. synthetic cannabinoids such as spice) only ever came to be because of current drug regulations - they were created because in many countries individual drugs are outlawed, so if you create a new one, it will be legal to sell it for a time.
The "traditional" drugs are more than enough for most psychonauts - why risk some novel substance with an unknown safety profile if you could legally and safely obtain high-quality cannabis, MDMA, cocaine etc?
I'd love being free to buy pharmaceuticals without the approval of my government and doctor; that's a feature, not a bug!
legalization is compatible with certain reasonable constraints. advertisement of any drug (maybe even FDA approved) can be prohibited. sale of unapproved drugs can be restricted to certain specialty stores so there's no risk of people assuming it's safe just because it's at their pharmacy or grocery store. you can mandate warnings on the packaging and/or at the entrance to the store. you can say no such stores within x feet of a school.
Do you just grandfather in a certain number of existing drugs (ie marijuana, MDMA, etc?)
What happens when a new substance hits town? The approval process for new therapeutic drugs is huge and very expensive - and only major pharmaceutical companies can generally bear the cost (plus the safety requirements are stringent enough that many currently illegal drugs wouldn't pass).
If you allow future street/recreational drugs to get a free pass (and tax them), you're opening a loophole for new pharmaceutical drugs to make it to market without due approval and safety checks.
Decriminalisation (Portugal-style) is a different thing altogether, but you don't get the tax revenues and the implicit sanctioning that brings.