Most heroin users have a number of reasons to prefer morphine and its esters over fentanyl, which has no legs, is dangerous, is not desirable because of the degree of euphoria as well as a je ne sais quoi that makes morphine the most euphoric of opioids, and many of its esters and ketones having that euphoria plus a more intense come-up
The original C R Alders Wright experiments with heroin in 1874 apply to all of the esters of morphine and codeine, since they came up with all of them in those experiments (although the first patent for dipropanoylmorphine was taken in 1972, they shelved all of their output as they were looking for some other property.. The limiting factor would have to be morphine availability, yet supposedly there are all these pharmaceutical on the streets, and this huge amount of opium . . . codeine, dihydrocodeine, dionine, hydrocodone, oxycodone, hydromorphone, and oxymorphone as well as dihydromorphine a;; have the same esters as morphine -- this opioid subfamily numbers in the hundreds. Some are more problematic than others, including the three of the first designer drugs from the 1920s: acetylpropionylmorphine has two watched precursors in addition to the controlled morphine, and dipropanoylmorphine has one -- these ones are controlled in the one case since it is used to make heroin and acetylmorphone, and the other is used to make fentanils and is watched for this reason.
I don't think drug dealers and their wholesalers with the exception of some evil and ignorant ones, want anything to do with synthetics like fentanils --if they are greedy, why take a chance on kil;;ing their clientele?