One of the big hurdles with full legalisation / taxation which you're suggesting (as opposed to decriminalisation) is how to deal with the approval processes for future substances.
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.