The cartels are not making money off of weed, or even relatively non-addictive psychedelics for which there is limited demand.
The kinds of drugs they distribute, especially since the boom in synthetics are very harmful.
We have an opioid epidemic that is growing rapidly, not unrelated to basic prescription use. This is happening even in places like Canada, where prescriptions are more tightly controlled.
While some highly conscientious slice of the population can handle some recreational use (i.e. a few burning man friends), even among them there are those that will fall (a highly-paid, professional colleague died of an OD), and among the general population large swaths will be immediately consumed by addiction were drugs to be easily available.
Decriminalization may have some positive effect, but if there are any restrictions at all, the Black market will still be enormous, with the 'legal' activity driving demeand for the illegal - probably the worst outcome.
The amount gained in 'taxes' would be a pittance next to the massive public health crisis, or even the lost wages and productivity. I suggest some material % of the population, say 4-10% would be tied up in this activity at least to some extent, with a large cohort of that group being very adversely effected, and many simply deal.
The social costs of widespread opioids/fentanyl, crack and meth would be disastrous. In the long run I think it would be worse than COVID, while it might entail fewer acute casualties, it would be a severe plague on civility that would be very difficult to unroot. If we increased addiction 20x via legalization, and then had to roll back legalization in order to deal with the crises, the 'cartels' that would appear would have 20x the market.
Especially since the advent of much more powerful synthetics which addicts seem to prefer and demand, there's little doubt we have no choice but to keep these things out of the hands of people. It's a game of Supply and Demand and controlling access points, if we really wanted to do it, I think we could.