Long term you couldn't kill more than existed, asymptotically the maximum number of ODs per unit time would be exactly equal to the number created, impossible to be more.

By allowing fentanyl to kill so many so fast we might be (almost certainly are) selecting for those who are less susceptible for whatever reason (less susceptible to addition, less susceptible to even beginning to go down that road, more surrounded by loved ones willing to act, more biologically resistant to the killing effects of ODs, etc.).