It is used on nearly every patient that has an operation with anesthesia and would be very difficult to find a replacement that is as versatile.
Fentanyl is given to nearly every patient prior to intubation and frequently redosed throughout most operations.
One reason why it is deadly in the streets is because a safe dose is so small and hard to measure: http://mediad.publicbroadcasting.net/p/wosu2/files/styles/x_...
They are even moving to take ownership of generic drug production:
https://www.nytimes.com/2018/01/18/health/drug-prices-hospit...