I generally agree with you, but I think decriminalizing is not only good first step, but does itself allow for a small amount of quality control. For instance if MDMA is decriminalized, but PMA (something you very much would prefer not to ingest but is nevertheless sometimes sold as MDMA) is not, then people holding PMA would still be at legal risk. Over time this should tilt the scales in favor of the decriminalized substances over counterfeits.