That's like saying that websites shouldn't have copyright violations on them, since every single piece of content uploaded to them can be manually reviewed and approved to prevent violations. Good luck with that.
The reasonble choice is to make the trade more difficult and raise prices. Cigarette prices have shown demand for some very addictive substances are far more elastic than some would believe.
I think there's better solutions than increasing exposure to addictive drugs. Non-addictive stuff I really don't care about. But they don't drive the drug trade either. I'm also for decriminalizing addiction to encourage people to seek help. Your suggestion is a bit too much anarchy for me
There will always be a way to get it into the country if there is a paying market.
And even with a perfect wall in place, we still let people through it and over it and around it via other channels. The problem isn't really just people physically walking across the border away from checkpoints.