They would take away the livelihoods of a large number of farmers. From an Afgan farmer's position, when there are these foreign troops with strange customs and behavior that you don't really understand or like, and then they destroy all your crops and leave you and your family to starve, it tends to turn you against them. And when you have a lot of anger and time on your hands, some will decide to take up arms against the troops instead.
A better solution would be to just pay the farmers more than they get for the poppy to produce something else instead. With some basic sanity checks on the amount of stuff they produce the inevitable cheating of the system should be somewhat manageable. It might not be very cheap, but neither is handling the effects of drug-based crime at home or posting troops abroad.
This (subsidised legal crop production) was suggested by Columbia in the eighties as a way to curb cocaine exports, the response: We don't make deals with Terrorists...
Found a number on the amount of money such a scheme would require: "Although Afghanistan's overall economy is being boosted by opium profits, less than 20 percent of the $3 billion in opium profits actually goes to impoverished farmers, while more than 80 percent goes into the pockets of Afghan's opium traffickers and kingpins and their political connections."
So 600 million dollars, give or take, to replace the economic incentive.
Sounds a bit unfair to all the saffron farmers in other countries. With half the land in Afghanistan suddenly devoted to saffron production, the value of saffron will plummet. Meanwhile, Afghan saffron farmers will be selling their saffron to the US government for some insane price. If I were a foreign saffron farmer I'd be pretty pissed off. Oh, and the number one saffron producer at the moment is Iran, so you'd probably get a vast industry in smuggling Iranian saffron across the border to be sold in Afghanistan.
Oh, and also the drop in opium production will raise opium prices, and so you'll need to keep raising the Afghanistan saffron buying price in order to keep it more attractive than opium farming.
I foresee all sorts of unforeseen consequences, and those are just the foreseen ones!
I agree with the problems you mention, and the likely unintended effects. My point was that it might be a least-worst-option alternative. I suspect there are no perfect solutions that can be imposed onto the Afghan people from outside.
If the could grow corn or soybeans - which are the main feedstocks for biodiesel - it would be better for everyone if they just used it for food production, as opposed to refining it and shipping it over here.
