Question becomes what is the handgun supposed to do? Is it a "low intensity conflict" gun, where you would probably use it if your M4 fails; to shoot down an unarmored suicide bomber as quickly possible, while firing as few shots as possible. If that is the case you probably want something bigger like a .45 or .40
Or is it a sidearm that needs to defeat some level of modern body armor that you are likly to see a Chinese or Russian "regular" equipped with? In that case I am not sure even the 5.7mm would be good enough.
This is kind of symptomatic of the military today: what kind of a military are we building and who is it designed to fight
PS: From a logistics standpoint; I imagine 9mm has a big edge not only because we have droves of it here in our stockpiles, but so do out NATO allies over in Europe.