That mass exodus likely has more to do with gunmakers practically giving away new .40s to departments to gain marketshare than any deficiencies with the 9mm. Glock did it with the .40 and later the .45 GAP, SIG-Sauer did the same with the .357 SIG. Any loss was more than made up since the 15-18 round 9mm mags could be sold at scalper prices during the Assault Weapons Ban.
With advances in bullet design I'd wager that any difference between the 9mm, .357 SIG, .40, and .45 is insignificant. Each has advantages and disadvantages, but they will all work for duty or defense.
Man, I sure do like you but we are not dealing with "mass exodus likely has more to do" but rather with facts.
Show me one, even recent, study comparing calibers that gives any advantage to the 9mm.
Almost everything is based on some proactive study. The speed limit on a roadway is based on the road width, traffic, commercial entry, curves and signals. The size of a traffic light and placement is based on the same things.
Now let's consider your "likely" theme. A 9mm round is cheaper to shoot so therefore a department with 600 guys is going to go through hundreds of thousands of rounds each year. Budgets are tight so why are they "likely" to buy a larger more expensive round?
The guns, at any price are not going to be swapped evenly for the old 9mm weapons. SPD had to pay $330 each plus the trade in. I doubt that could be considered "practically giving away" or even close. The depts across the US went to guns that gave the officers some advantage.
Now to some history lessons for you young starters. Back during the 70's, the perps were buying the foreign 9mm guns with high capacity mags like the Mac 10 and others. Officers on the streets were carrying larger calibers but with only six rounds and they felt outgunned since most cops are not good shots. Departments went to the guns that worked the best while having the large capacity mags. This meant a 9mm as that was all there was in quality products at the time.
With some experience in street use, they found the 9mm to be lacking. A new gun with a 17 round mag was now available called a .40 cal. It was topping the charts in efficiency and stopping power. Depts went to it in droves. Now there are double stack mags for the .45acp and some depts are going there. True, there are some departmental holdouts like NYCPD (although I am told this is slowly being changed) due to budget concerns. Having several thousand officers takes millions of dollars that NYC does not have and taxpayers are not willing to fund. LAPD is also changing over but the cost is to the taxpayer and it takes time for a complete change over.
Again, those defending the 9mm likely only has a 9mm to use and that is fine. For those with options, they will carry a larger caliber. For me, with a room full of guns, I can use any caliber since I have multiples of just about each caliber but I chose the .45acp as my primary carry, a .40 as a second choice, a .357 mag revolver as a third and the 9mm as a fourth. My decision is based on street experience, studies and seeing the experiences of others first hand.
Now when are we going to lunch again?