It's just another common defensive/service pistol caliber. There's always been an ebb & flow to these sort of things.
Yes, there's been some resurgence of interest in 9mm among LE circles, and the reasons have normally included
disparate impact types of training issues (felt recoil & controllability, accuracy, etc), as well as cost & availability of ammo and maintenance of service weapons (wear & tear with harder recoiling calibers).
Not exactly a surprise.
Personally, I wasn't interested in the .40 S&W until it had been in-service for about 10 years. That was when I was told my agency was going to be introducing a small number of them into general service with our 9's. I picked up my first one at that time, and shortly thereafter was issued my first .40 duty weapon.
Over the years since then I've bought an additional 5 pistols chambered in .40, and have carried 2 different models of issued .40's (along with the other 9's & .45's).
I learned to appreciate the .40 after many ten's of thousands of rounds fired, and nowadays I find it rather equivalent to shooting my 9's (felt recoil, controllability, recoil management, etc). I didn't feel that way for the first several years of using it, though.
I've helped train folks who complained of a significantly noticeable amount of increased felt recoil (.40 versus 9 or .45), and others who claimed they couldn't feel anything different. Well, felt recoil (and how folks feel it)
is rather subjective.
In all the various armorer classes, listening to other armorers and gun company armorer instructors, it's not been uncommon to hear that the .40 (and .35SIG) is generally harder on guns than 9/.45.
No surprise, and no big deal ... TANSTAAFL.
Anyway, now it seems we've come most of the way back around the circle in LE. We're not returning to revolvers ...
... but the venerable 9mm has been given a new lease on life with the better designed & performing hollowpoint bullets and loads of recent years (and more to come).
Now that 9mm is once again among the approved calibers for personally-owned duty weapons at my agency (and new issued weapons chambered in 9, .40 & .45 are in the works), I've seen more 9's making an appearance at quals.
Even some longtime, died-in-the-wool .45 ACP users are appearing among the ranks of folks carrying 9's. Not surprising, as someone who can shoot a .45 well can probably shoot a 9 even better, meaning faster and controllably. Obviously, comparing 9 & .40 has often been yielding the same observations.
Hence, I'd not be surprised to see the small fed agency that is the FBI (13K armed?) return to using 9's.
Now, if the 250,000-odd armed folks who fall under the umbrella of DHS decide to transition from .40 9and .357) to 9? Might be noticeable.
Might be even more .40 starting to hit the store shelves.
Probably more used .40 pistols, too, as the inevitable followers of such things decide to emulate .gov users.