I don't think the 40 S&W would have taken off without the FBI doing the testing and adopting it.
A lot of depatments adopted the .40 call because the FBI led the way, in some ways put pressure on departments to follow suite, and of course made it somewhat safe to follow the FBI - if anything ever went wrong they could point to the research the FBI conducted and the conclusions they came to. On the other hand, if they stayed with 9mm and some officer got killed after putting five or six shots into someone - they would have had a long line of detractors asking the department why they didn't learn from the 1986 Miami shootout, and why the department thought they knew better than the FBI when it comes to caliber selection...
With the Miami shootout, the FBI developing their protocol, and adopting the new 40 S&W it caused it to take off. That is the power of the FBI.