If amphetamines were legal we could get a lot of people to switch away from black market meth into cleaner and safer amphetamines in better controlled doses though. With most amphetamines being illegal and the fewer medical amphetamines being highly controlled and restricted to limit supply, you guarantee people are going to be taking black market street meth of unknown quality and type. There is a reason prescribed adderall has 4 different amphetamine salts in it and not just one random one, and is a large reason why problems with adderall abuse are so less common and severe than problems with unregulated meth abuse.
If your goal it to have zero bad effects from people using drugs, and you think you can achieve atleast close to that, then prohibition does seem like an ideal policy. But if you believe a decent number of people will find or produce and consume drugs regardless of the law or enforcement capabilities, as evident by the last 60+ years of failed strong illegal drug policies, prohibition drug policies for any drug is silly and leaves you almost no room to educate people about drug usage or how to minimize harm and addiction and limites their choices in being able to pick the safest versions of a drug.
If your goal it to have zero bad effects from people using drugs, and you think you can achieve atleast close to that, then prohibition does seem like an ideal policy. But if you believe a decent number of people will find or produce and consume drugs regardless of the law or enforcement capabilities, as evident by the last 60+ years of failed strong illegal drug policies, prohibition drug policies for any drug is silly and leaves you almost no room to educate people about drug usage or how to minimize harm and addiction and limites their choices in being able to pick the safest versions of a drug.