There is not a huge incentive to change pistol calibers for a couple reasons:
9mm is a NATO standard.
Most potential pistol rounds are going to be ineffective against body armor anyways, and most of the world's forces now wear body armor. So the only real difference would be when shooting civilians, like when imposing police states and fighting insurgents and not professional forces.
But gearing your troops to fight insurgents at the expense of what would be best at fighting professional forces is quite dangerous. You never know when we might be actually fighting a real military.
Effigy said:
Since they're required to use FMJ, I think switching to .45 makes the most sense.
Against civilians without body armor, like many insurgents this is true.
However they already had more .45 ACP pistols than they would have ever needed, and president Bill Clinton had most of them destroyed so they could never possibly end up going from the massive armories and into the hands of US civilians. (Just like he had fare more m14s destroyed than we have infantry in the military. When exactly such longer range rifles would be in high demand in Afghanistan and can be modernized to better fill such roles.)
So if you want to talk about throwing away money, destroying hundreds of thousands of .45s only to determine they could use a good .45 would be a great example.
It would make them look so unbelievably foolish that I think they would avoid a new .45ACP gun for just that reason.
The .45 ACP's ballistic coefficient and low velocity also makes it really hard to create rounds that will defeat even low level body armor, so if facing armored threats they could not issue ammo with better penetration like they can for some other calibers such as 9mm.
And finally a pistol is just jewelry most of the time, most soldiers on the battlefield have much more effective rifles and machineguns.
Battles are not won or decided by pistols. Only a small number of units actually use pistols frequently, and many of them are not limited to the standard issue sidearm and so it doesn't matter what the standard issue sidearm is.
Investing billions of tax payer dollars in something that plays almost no role in battle to purchase a bunch of pistols that do virtually the same exact thing as something they already have is just wasteful.
Doing the same thing to purchase guns that also screw up NATO supply lines for no real increased performance is just foolish.