What? Prohibition historically showed the exact opposite.
I suspect higher purity & potency of street drugs has much more to do with more sophisticated operators operating outside of the US than strict prohibition. Same with fentanyl.
I believe OP wanted to make the point that one of the most important things for people profiting from the illegal sale of drugs (meth or heroin/opiates) is to minimize the amount that has to be trafficked (1kg of 10% meth vs 100g of pure meth or 1kg of heroin vs 10g of fentanyl).
Prohibition encouraged higher potency but purity was highly questionable. The stills themselves often leeched contaminants from the metal used to make them, as well as the heads and tails of the distillate, which should be discarded but might not be.
