Just try to picture what the situation would be without it. And the sentences doesn't seem quite harsh.
OTOH, look at what Singapore did - it seems to have fixed their problem. The "mixed message" in the west about not prosecuting casual users in some place is certainly also not helping.
(and I'm saying that as someone who believe there is no point in restricting what someone may or may not inject in its own body, but if one was seriously against drugs, the current situation would seem like a feeble response to a real threat)
"Just try to picture what the situation would be without it"
It is not hard to picture, because drug prohibition began a century ago in the USA. Back when drugs were legal here, things like the Industrial Revolution happened. Society was not some kind of pit of despair when drugs were legal; we had problems, sure, but we still have problems today and the drug war has not solved any of the problems we had previously (but it did introduce many of the current ones).
"the sentences doesn't seem quite harsh"
Jail time is a harsh sentence. America is so far to the right these days that we tend to think that putting someone in prison for "only" five years is not harsh.
It is not just about the sentencing. Right now, we have tens of thousands of paramilitary raids on civilian homes each year; property is damaged, dogs are shot, and people are killed. These raids are not reserved for the most dangerous criminals; they are routinely used to serve search and arrest warrants against people who are either unarmed or only lightly armed. Many thousands of innocent bystanders have been killed by the paramilitary teams that carry out these raids, as have many thousands of innocent people whose homes were incorrectly targeted.
Worse still, the very paramilitary forces that carry out these raids are allowed to keep the proceeds from the sales of property seized in the course of their duties for their own department budget. Some police forces have become "self funded" as a result. It is a case-study in slippery slopes.
This has not helped to solve our drug problems. Not only are people still using drugs, but the drugs they are using are causing unnecessary harm. Methamphetamine is not necessarily a destructive drug; it actually has medical uses (as a treatment for narcolepsy, ADHD, and for appetite control), and is sold by the pharmaceutical industry under the name Desoxyn. Yet people who want to use this drug recreationally cannot buy it legally (it is a felony to transfer it to someone who lacks a prescription and a felony to prescribe it unnecessarily) and wind up buying it from black-market sources, receiving a product that was produced under poorly controlled circumstances and which is frequently adulterated with the byproducts and waste from the production process (these cause far more harm than the drug itself; it is these adulterants that give methamphetamine abusers their "sickly" look). Likewise with opiates: heroine is often "cut" in ways that are poorly controlled, and on at least one occasion a black-market opiate dealer sold MPTP, a drug that permanently induces Parkinson's disease, after failing to properly manufacture the opiate MPPP.
So if someone were not on some moral crusade and actually wanted to address the problems society has with drugs -- someone like me -- they would support a complete repeal of the war on drugs and an entirely different approach. We manage to produce and distribute alcohol in a regulated manner. We don't need to throw people in prison, we just need to ensure that people don't kill themselves or anyone else and that if a person needs help, they can get it.
OTOH, look at what Singapore did - it seems to have fixed their problem. The "mixed message" in the west about not prosecuting casual users in some place is certainly also not helping.
(and I'm saying that as someone who believe there is no point in restricting what someone may or may not inject in its own body, but if one was seriously against drugs, the current situation would seem like a feeble response to a real threat)