> "New drugs already exists that are more addictive and powerful than old ones. Like Fentanyl, for instance. Who knows what research labs would come up with if they threw lots of money and brain power into it?"

Taking the perspective of a business man, it's possible for drugs to be too addictive, as they have the bad side effect of killing your customers, which is bad for revenue and a PR disaster. So even if a 'designer drug' market emerged, I don't think you'd find many companies that wanted to develop something as addictive as fentanyl.