Just because fentanyl has a shit therapeutic index doesn't mean it has a high body count (it gets used just fine even in tandem with other depressants in clinical circumstances...) - I would say the problem is more in the hands of the tough-on-crime lawmakers who seem to lack, at least in my neck of the woods, remorse.

Also, look up sufentanil, and once you feel you're ready, carfentanil..

Also, yes, yes, fentanyl is made because of its potency and therefore cheapness, but a napkin-back calculation shows: 17 $ / g codeine * 100 g codeine / 70 g morphine in a conversion tek * 3 g morphine / 2 g heroin = ~$37 g/heroin, if you were a pretty good chemist, and could legally source precursors (pyridine HCL, acetic anhydride..), which is probably a lot better than the stepped on shit you would get for even 60 bucks a gram (although you can get 60 dollar grams on AlphaBay iirc). That's near a dollar a dose for an opiate-naive user and a day's worth for the 60-year old junkie down the street.

Also, the high/euphoria from fentanyl is much shorter lasting (hence its appropriate use in surgery) - I could see this encouraging heroin users to use more under the idea that they've simply increased their tolerance..

Also, Prince died with traces of U-47700 in his system.