Coming from a guy that shoots .40 primarily, NATO should stick with 9x19.
It's a sensible cartridge. Handguns don't win wars, and most of the world's militaries are most familiar with 9x19. In addition, it's light to carry and easy for beginners to shoot, while having decent power and great capacity.
13 Pistols, 5 Rifles, 1 Shotgun, no time to shoot them