For that matter they could just build a catapult to fling the goods over the wall to their contacts on the other side. Or fly it over in light planes, gliders, drones... The Mexican people are very resourceful. Hence the reason when their pot market was flooded over here, they immediately went to opium poppies.
I agree with your comments about alternative means of delivering weed -- with emphasis on drones. But I strongly disagree with your theory that heroin is replacing marijuana. These are two very different drugs and they appeal to two very different user mentalities.
I used marijuana rather regularly during the 1970s. So did my late wife and most of our friends -- not one of whom would consider using heroin. While I can't account for the increase in heroin use I absolutely reject the notion that it has anything to do with marijuana.
Well, you're wrong. It's more about the Mexican economy being dependent upon illegal drug sales in the US...and how the Mexicans are resourceful. Who gets hooked on what in the US is only a secondary concern and certainly not the quick solution to the heroin epidemic. The solution is to understand the politics of why the heroin imports increased dramatically when pot became legal here.
It goes to follow that cartels know a depressed populace in the US might want a quick and easy copout. They know if you build it, they will come... And with Dr.s playing their role paving that social-acceptance niche, overprescribing opiate pain meds to the soccer moms and dads, the Mexicans just took over and rode that wave on its natural course anyway.
In short, the heroin epidemic is the Mexican quick-fix to their crippled economy when pot became legal here. The heroin epidemic is purely a child of politics. The Mexican penchant for passive aggression...
"Ok gringos, you want to flood our pot market? We'll just sell your kids heroin instead!"