If the purpose of the law is to make psychoactive substances illegal, what would be ridiculous it would be to ban them one by one as they pop up (and they will never stop popping up)
That is, quite interestingly, what Germany is doing with LSD-like substances. They banned, in this order with 1 year between, 1p-LSD, 1cp-LCD, and 1v-LSD. 1d-LSD is currently (openly and seemily legaly) sold in Berlin.