Rubbish. Folks made the exact same arguments about alcohol and marijuana. Specifically with alcohol, anyone can walk into a treatment center without fear that they'll be arrested for the mere _use_ of a substance. (Marijuana has very low risk and rates of addiction, physical or psychological.) If "hard" drugs are legalized, then they will likely be treated the same as alcohol and pot and tobacco: highly regulated, so…
Let's take fentanyl for example. It kills so many people that there isn't any good reason for it to be a legal drug off prescription. The difference between fentanyl and other drugs is a matter of degree.
Heavy users vastly prefer heroin, casual users much prefer oxy or similar. Fentanyl is just cheaper to obtain and easier to traffick.
Furthermore a lot of fentanyl deaths are caused by mislabeling and misidentification of the drugs which would not happen without prohibition. Fake painkiller pills with fent in them, fentanyl poorly mixed into heroin or filter, etc.