Errrrr no.
The price of heroin crashing definitely increases use. Being a much harder drug maybe it will impact less, but I was just reading that drug addiction and overdose deaths now exceed traffic fatalities in the U.S. and that was partly attributed to a dose of heroin now costing “about as much as a six pack of premium beer”.
Still, legalizing Marijuana use is definitely a net good, but there may be some unforeseen consequences.