The argument that you are providing is very simple but it doesn't address a very complex problem.
There are factors that effect the poppy eradication program.
1. The Afghan Gov't has forbidden ISAF or Coalition Forces from getting involved in counter narcotics. I can tell you for a fact that many/most of the Afghan gov't is corrupt or receiving kick backs for this.
2. The poppy farmers are not the ones making all the money it is the smugglers, Taliban or Al'Queda. In many cases it is the land owner that is forced to grow poppy by these groups.
3. Your assuming that we (ANSF or ISAF) have control over the areas that are growing poppy which is not the case.
My recommendation for solving the problem:
1. Have the Afghan gov't subsidizes alternative crops (corn, cotton, grapes, etc...) that are traditionally grown in Kandahar.
2. Ramp up the Poppy eradication program and add ANSF security.
3. Have the Afghan gov't allow for ISAF and Coalition Forces to assist in poppy eradication efforts.
4. Create a counter narcotics reporting incentive program. (Local Nationals get paid for tips that lead to arrests)