1. Afghan opium products do not come to the USA. They go to regional countries - especially Iran - and to Russia. US illegal opiates are grown in Mexico and South America.
2. There are 19 countries licensed to grow poppy for legal drugs. These licensed farmers are not producing even a tiny portion of their capacity because there is not enough demand.
3. The world price of legal products is a few dollars a kg, about 5% of the price the Afghan farmers gets. Afghan farmers will not accept the world market price and the bad guys will always outbid.
4. The talked-about shortage of opiod drugs is manufactured from two non-related streams of discussion and is not, in fact, supported by the drug administration bodies of world governments, the legal bodies who would actually be managing national drug supplies.
5. There are loads of other things that Afghan farmers can grow that are more profitable than poppy but there is no access to the marketplace because the roads are not safe and farmers have no transport. The bad guys come right to the farm to pick up the harvest. Afghan opium is a product of the conflict and would not be profitable otherwise.
Read David Mansfield, Pierre-Arnaud Chouvy and World Bank publications in addition to those of UNODC for more information.