There's been a trend that seems to have been developing within the LE field, which is the "return" to the 9mm.
Reportedly, the papers to authorize the program to transition the FBI from .40 back to 9mm is sitting on someone's desk, awaiting final approval. The largest PD still uses it (NYPD) and the nation's largest SO (LASD) is remaining with it, although replacing their aging inventory of Beretta's with M&P 9's over the course of the next 5 years. Texas DPS is going to be issuing M&P 9's to new recruits, and allowing existing troopers to either keep using their SIG .357's, or switch over to the M&P 9.
Expect to see more agencies going back to 9's for their own reasons.
Also, it's not been uncommon for some time to have heard how used .40's have been easier to find than used 9's on the commercial market.
I finally started to see this occur at my former agency once it started allowing folks to buy their own duty weapons from an approved list of 3 major manufacturers, and could choose either 9mm, .40 S&W or .45 (up until now the choices had been either .40 or .45).
I've even seen some very experienced instructors, who were long time 1911 enthusiasts & users, start to switch over to 9mm weapons. The answer was usually as simple as that for
duty purposes, the higher capacity of the 9's was appealing. (These sames folks usually still chose to carry lower capacity, smaller weapons off-duty, though.)
Although I'd been required to carry either an issued .40 or .45 prior to my retirement, if I'd been offered the chance, I'd have continued to have carried that 6906 I had to give up several years ago.

(FWIW, I was carrying an issued 4513TSW at the time of my retirement.)