Ditto with smee.
The army does not train pistoleros.
People on this forum (and this section of the subforum in particular) are adherents to pistolcraft. 9mm is small to us, .45acp and 10mm are not difficult to control, we have years of handgunning experience and have sent hundreds of times the number of rounds that the typical army recruit gets prior to being issued an M9.
The M9 is not a bad pistol. The 9mm is not a bad round.
It killed plenty of GI's when held by Germans in WWII, and to this day we have a very healthy respect for the Luger and Walther P38 pistols.
Insurgents in Iraq and A-stan have a healthy respect for the GI today who points a Beretta M9 at him.
When you need a pistol in a multi-attacker armed confrontation... you need a lot of rounds in it. Increasing diameter reduces rounds.
The 9 is fine.
Move on. Even the infantry rifle is not being seriously considered for replacement, let alone the infantry pistol.
Back when calibers were larger for our troops, our troops came from backgrounds where shooting was more common.
What we need now, is greater marksmanship. Not bigger bullets.
A miss with a 9mm or .223 would most likely be an even bigger miss with a .45 and a .308.