Who do you pay to grow something else? It would have to be every farmer in Afghanistan since once you start paying the ones who are actually growing Opium, the ones who haven't been growing Opium will start growing it. The ones you are paying may stop growing Opium and start growing Marijuana. Or they may tell you they have stopped growing Opium when they haven't. If we knew every farmer who was growing it already we could presumably stop them without paying.
One other fact, even if we made every farmer in Afghanistan multi millionaires so they could retire to the south of France and spend their days in their villas, new farmers in Afghanistan and elsewhere would step in and grow Opium to fill the demand. If there is a demand for a product, someone somewhere is going to supply that product. It's market fundamentals of supply and demand.
That is why legislating black market products like drugs out of existence hasn't worked and will never work. The legislation is an attempt to change human nature, and human nature is a natural law, one that cannot be changed by legislation.
Invest a few bucks in some private (read: well armed) 'contractors' to protect their investment, and the drug lords could be run out of business.