The Portuguese case study says that it's really not a bad idea (though not referring specifically the increasing or diminishing illegal business, just less "street value") [1][2]. But I agree with your corollary.
[1] https://www.drugpolicy.org/sites/default/files/DPA_Fact_Shee...
[2] https://en.wikipedia.org/wiki/Drug_policy_of_Portugal#Observ...