The more we have to pay for each other's healthcare (insurance costs going up, ACA subsidies, maybe a future "single payer" situation), the more other people's health decisions are going to be our business.
Oh! so we should be supporting fentanyl killing people because it makes our healthcare costs go down because now they're dead NEETs instead of a drain on society? how modest.
By the societal benefit model, yes. By the more libertarian model yes because of bodily autonomy. Either way yes. Trying to protect them from themselves is worse from virtually any angle and especially the humanitarian one.
Technically, that is only if fentanyl is disproportionately killing young people who are already addicted due to some other cause and not likely to be economically productive again.