First, as the author admits as having some logic, "If someone can vote, if they can join the army, fight and die, get married, then they should be allowed to buy alcohol." When does adulthood kick in? If you couldn't join the army until you were 35, get married until you are 35, or own a gun until 35, I am sure the death or divorce rate statistics would go down, but nobody wants 35 to be the age of adulthood.
Secondly, this claim: "Formerly wasted talent would be used in the economy, countless lives would be saved, and families and relationships would be healthier and less violent." The author really doesn't want to talk about these claims were lifted straight from Prohibition and the WCTU, verbatim. It turns out humans who don't care about wasting talent like wasting it any way they can; be it alcohol, drugs, video games, gang violence, take your pick. The WCTU learned that the hard way when gang violence for alcohol became the new favorite way to waste talent.
Third, he follows the premise that dangerous means we should discourage it. I don't actually agree. Motorcycles are 20 times as deadly as cars to the rider - but I don't believe the government has any role in shutting them down. There is also a very high risk of unexpected consequences - for example, the requirements for lots of new safety equipment in new cars has been shown to have significantly priced out poorer communities, causing them to opt for cheaper, older, even more dangerous vehicles.
Fourth, I think is honestly stupid that he factors drugs in as an example of how "we can ban drugs - why not alcohol?" Has literally anything about how drug bans have been enforced gone well? We can think drugs are bad for people while admitting the War on Drugs has been a nightmarish disaster with over 19% of High Schoolers admitting to having used drugs in the last 30 days, not to mention causing considerable racial effects, as well as having built a logistics service that rivals Amazon while being illegal - and yet he brings it up as though it was a good example!