Here's an interesting take on the selection and use of the .40S&W by the FBI. Written by one of the participants in the 1986 Miami Shootout and observer of the testing which led to the creation of the .40, Edmundo Mireles, author of "FBI Miami Firefight: Five Minutes that Changed the Bureau."
https://gundigest.com/tactical/1986-miami-shootout-the-aftermath
To me, the question has nothing to do with "caliber wars" - that's kind of a childish argument to get into and I stay away from childish arguments - but it is all about marketing, doctrine and training. Nothing much of real significance has changed about the 9mm bullet in the last thirty years. Small caliber hollow points are still unreliable, sheetrock and windshields still defeat small caliber projectiles, velocity is still secondary to mass in the force equation (F=ma; always, everywhere in the known Universe) and bigger bullets still cause bigger wound channels and bigger holes in things. The .45ACP is still the most effective man-stopping, manageable, and commonly-available pistol cartridge ever issued; but, it is a big, slow-moving bullet which is not affective at punching holes in engine blocks - and for some odd reason, the FBI is fascinated with the idea of stopping cars with handguns by shooting through steel doors and engine blocks. Also, any concealable .45 is going to carry fewer than seven rounds, an insufficient number for a field operative, and a lot of small-framed agents cannot shoot a .45ACP accurately - their hands are too small for the grip. The .40 was chosen because it came closet to the .45's performance in a small, manageable package. And it could punch through a windshield without deflecting or losing significant mass.