This proposal would give street users a break (a step in the right direction) but would leave the supply chain intact. Cartels would continue to destabilize regions of Mexico.
Your objection is correct, a fundamental issue with "decriminalisation of personal use" is that it's still quite trivial for officers to charge "possession with intent to distribute" for people who don't make a business out of dealing drugs. Talking about giving drugs to a friend / having a scale (to measure your own doses) / whatever else -- the police still have plenty of ways to threaten people with hard time to get you to plea guilty to a lesser offence. Hell, people who share drugs have been prosecuted for homicide after someone OD'd. It's still prohibition, just with better PR.
Such a state of affairs still cranks the iron law of prohibition (https://en.wikipedia.org/wiki/Iron_law_of_prohibition, the reason so many people die of fent overdoses), this still leads to police having legal authority to stop and search your car if they claim to smell weed, it still leads to people involved in the drug trade having no options for dispute-settlement outside of violence (https://www.washingtonpost.com/opinions/how-the-war-on-drugs...), it still leads to police focusing on the impossible task of ending the drug trade to the point of trashing the homicide clearance rate (https://www.themarshallproject.org/2015/04/02/why-are-americ...).
Without prohibition, drug trafficking operations have to compete with fully licit container ships and will get annihilated; because the illicit drug trade's transport methods are optimised to evade detection and without that selection pressure, there's no point for all the violence and the concealment tactics.
People who could afford it built extensive wine cellars and stocked them up before Prohibition began.
The head of a household could make up to 200 gallons of wine per year for personal use and to share with family and friends. San Franciscans bought their grapes off the train that came down from Wine Country. If you didn't want the mess of crushing the grapes yourself, Golden Gate Grape & Juice Co. would crush them for you and deliver them to your home in a barrel from Pigiatura Completa Dell'Uva next door. (Scroll up and down in the first link for some photos.) [1] [2]
If you didn't have a cellar or a source of grapes, you could mail order a "grape brick" of compressed dried grapes that came with a warning label: "After dissolving the brick in a gallon of water, do not place the liquid in a jug away in the cupboard for twenty days, because then it would turn into wine." [3]
You could buy liquor for medicinal purposes with a doctor's prescription. The prescription listed the type of liquor required, how much you needed and how often to take it, and you would get the prescription filled at your local pharmacy. Walgreens grew from a few drugstores in Chicago to a nationwide chain by supplying medicinal liquor during Prohibition. [4]
Clergy could buy sacramental wine and share it with their congregation for church services and other needs. California's Sebastiani Winery, founded in 1895, survived Prohibition by supplying sacramental and medicinal wine. [5]
Daniel Okrent has written extensively on Prohibition. I highly recommend his book Last Call and related talks he's given. [6]
[1] https://books.google.com/books?id=vqFxq6nv4rEC&pg=PA104-IA6&...
[2] http://www.sfgate.com/bayarea/nativeson/article/Prohibition-...
[3] https://vinepair.com/wine-blog/how-wine-bricks-saved-the-u-s...
[4] https://www.smithsonianmag.com/history/during-prohibition-yo...
Adult drug users are not able to obtain their drug of choice safely at a reasonable cost. This creates low-level street crime like car break-ins and dead bodies on the street due to impure drugs laced with other drugs like fentanyl and carfentanil.
It's much more difficult for minors to buy alcohol than illegal drugs. The reason is because the alcohol market is regulated (probably too much, but besides the point) and laws restricting access to minors is heavily enforced.