The peacetime military wants 9mm. The wartime military wants .45 ACP. Just because NATO likes it is not a good enough reason to use it.
As with anything guns, everyone has an opinion. As both a firearms instructor and having some connections with law enforcement professionals, I am informed that modern defensive ammunition for the 9mm is as effective as 45 ACP. Now I dont know if the military is still restricted to ball ammo or not for combat with the handgun. The 9mm is less weight than the 45 ACP by up to 100 grain or more, but the muzzle velocity is also anywhere from 400 to 600 ft/sec higher. the 9mm is smaller and lighter than the 45 ACP so you can carry more ammo, and mag capacity is more. So like everything, its a trade.
The 1911 in 45ACP is still a good handgun. But the P320 is as well, and has a lot of things going for it now that bugs have been worked out. The Beretta is replaces was a POS if you ever had to use it in a dry dirty environment. Besides being prone to malfunctions from seizing up with dirt or just not enough lube, the M9’s open slide design is everything you dont want for pinching/nicking skin when dealing with malfunctions.
Goofballs like me think they’re both wrong and that 10mm is still best millimeter...