@gollum,
The sellers of fentanyl (the ones who are causing the ODs) are generally drug dealers, sold on the cheapness of the product as another user stated, to cut with other products and make themselves more money.
In this scenario, the deaths can cause an increase in uptake, rather than a decrease, since it makes their product seem more edgy and dangerous. Drug users are often easily sold on things being "cool" if they're deemed 'dangerous' and are happy to play Russian roulette because it will give them a greater rush.
The supplier of the fentanyl, some say it is Chinese State, to the drug dealers on the other hand, benefit from both the drug addictions and the deaths, if indeed they are trying to destabilise a rival country. At least, that is the prevailing view from commentators who sit on this side of the issue.