This argument always seems a bit silly to me.
Perhaps these people oppose legalization in part because they don't want more drugs to be as common and socially acceptable as alcohol and all the evils that come with it, and simply regard alcohol and tobacco use as a largely lost cause. This argument is common and it always frames the issue as if the person is 100% pro-alcohol.
Fact 1 - lots of people, maybe even most people in the western world, use alcohol to varying degrees and have jobs and families that are not falling apart due to their drug use.
Fact 2 - many currently illegal drugs are less severe in their acute effects than alcohol, are less severe in their chronic effects than alcohol and are less addictive than alcohol.
Combined, these torpedo the point stated in the article, but not your point. The only way to address your concerns there is with studies of what happens in places like Amsterdam and Colorado.
Personally I would argue that even if legalising drugs makes some things worse (more users, more addicts), applying the cost of the drug war to mitigating the harms instead of amplifying them is a win regardless.
It's a long article, and it contains ungrounded assertions like this -
"Addiction to, or regular use of, most currently prohibited drugs cannot affect only the person who takes them—and not his spouse, children, neighbors, or employers."
This is ... well it's just wrong. Many people regularly use alcohol in moderation, there's no reason to think that regular, moderate use of cannabis and various other currently illegal drugs would be any worse when alcohol has one of the worst harm and addiction profiles of any drug.
So I'm not convinced I want to take the time to answer it, as it's not really honest.
--edit-- I've read more of it now, it's full of preconceptions and assertions like this one, no grounding in facts. It's not a good set of objections.