I agree that it is easier to monitor terrain technologically speaking, but it still hasn’t helped us find Bin Laden or decapitate the heads of the Taliban in a surgical strike. Nor does it help with manouvering troops around, except possibly by providing air support.
We could buy the poppies for pharmaceutical use, but that would be a lot of morphine. I am not involved in medicine, but I don’t hear a lot about a shortage of morphine.
Perhaps a carrot and stick approach would be best; “If you grow crop x instead of poppies we will give you money, if not we will burn down and salt your poppy fields”.