I'd argue that for most customers the various drugs are somewhat feasible substitutes for each other, many of them fill similar needs. The introduction of new "products" seems to be driven by supply side because they're easier to manufacture or transport, because some chemicals aren't yet illegal, etc.

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.