Glad you started this here, didn't want to be the one for fear of being a one-note Johnny .
I find it odd ( but not surprising) how much focus goes into the selection of a sidearm for the military . It is statistically not used in terms of how many people actually get killed with the thing in relation to mortality from war in general . Maybe slightly more than the bayonet , but not much . The controversies surrounding the adoption of the Beretta in the early 80s were epic , much ink was spilled, and congressmen that don't know a bullseye from bullshit had their full say on the issue . If you didn't know better , you'd think it was Facebook or something important .
But actually it is important . A pistol, like a bayonet or even skill at unarmed combat , is a confidence builder . Not in a " " sort of way . That is something that people who have seen a lot of TV and have never been in harms way for extended periods of time might think. But the fact of the matter is that when you have things seeking your life , it is comforting to know that there is a base level of preparation that is always with you ( as a pistol belt probably should be for a soldier, even going to the loo) that will allow you to fight your way to an adequate defense . Having that sense of not being quite helpless no matter what compounds with other skills and mindsets and allows one in sum to do things that one otherwise might not. The handgun is carried against the possibility of encounter , and is no more cumbersome, traumatic or strange than having a cell phone or a fire extinguisher in the kitchen .
As well, for some soldiers , a pistol is a required piece . Sennacherib put up a poem about the tunnels in Vietnam , which were often so tight that even a pistol could only be employed with much difficulty . In any sort of a CQB situation, when the primary weapon goes down, you have little other choice than to transition to a backup ( guns go down when you are using them, which usually means the other guy is using one, too ) . Clearing a malfunction under fire is, I am told, one of the least pleasant experiences one can have in terms of stress levels ( " worst sound in the world is a click when you want a bang" - Jeff Cooper ) .
As to something to replace the .45 ACP, I am not sure that the 20th century didn't see the arrival of some kind of point of diminishing returns . Unless there is some huge leap made in technology, there are only marginal improvements to be had for tremendous investment . How much better can we do than a 14 shot .45 ? Can we get much more lethality , lighter weight and usability than an AR at a cost that will be worth it ?
One of the biggest problems I have with writing scifi ( and I am trying it right now) is envisioning where weapons development is going to go . Twenty years can be done believably. Fifty, no.
Oh , well. Here is something I am thinking of getting as a " Ah, shit" gun.