Super late to the party and I am sure it discussed but what the hell.
1) Operations have changed QUITE a bit.... We do still have open field operation but many engagements have moved to Urban built up environments. The Vietnam era and WWI storming beach and heavy Artillery days are much less due to precision strikes and surgical troop movement.
2) With changes in ROEs and the need to limit innocent casualty. A close encounter NOW days are more prevalent to positively engage targets, rather than RANDOM Artillery strikes. Again precision guided bombs and Drones with Target identifiers used in greater number. With on the ground Troupe movements doing more and more Urban environments required the up close and personal engagements now days.
So while we say you are too close.... the battle field has change to much nearing engagements.
With that a 14.5" AR system, with a 3200fps projectile with an effective range of 500meters
Compared to a 4" pistol system with a 1200fps projectile in an indoor urban environment. the limitations on penetration, size, movement and accuracy can change. To bring up a 14.5" barrled AR and aim at a target 10ft in distance compared to a 4" barrel System with a specified round that can fragment and limit over penetration. Could be an evolution of urban combat.
finally is the evolution of the system itself.
M9 being a double /single action system, that is full metal with a specified grip angle and girth. Limits the ability of the soldier unless there is an IMMENSE amount of training
M17 now being, polymer in base design, multiple grip configurations. allows the shooter to have a more ergonomic and comfortable pistol. THIS can increase accuracy and decrease fatigue in long engagements effectively increase the ability of the solider to be accurate and precise while limiting potential innocent bystander casualties.
Agreed that a sidearm is primarily a defensive weapon, but it can be offensive too. Example: clearing out VC tunnels with a .45.
In all fairness, MOST weapons in the armory are ALL self defense weapons. NOT just the side, What I mean is ROE's dictate what your ability to engage is. So a rifle is a self defense weapon as much as a Tank, AC130. If the threat escalates that requires return fire. that is the justification. MOST times out side of black ops...... US Soldiers do NOT Engage a target unwarranted as that is NOT in the ROEs to do so.
Tunnels is a good example of what potential battle field evolution happen to our CURRENT Urban operations. But lets be fair.
Colt 45 vs a M17 9mm. carrying capacity. Recoil and handling, penetration and collateral damage. This is one of the reason why the went to a 9mm system in the first place.
So I know we are agreeing I am just edifying. That a pistol is a secondary weapon. Until used for a specified mission that requires a short barrel system. With that unfortunately the line combat operators do not have full access to an Armory to include much more common or specialized short barrel systems so a pistol like the M17 should hopefully meet in the middle for those on the go operations.
"It's not a job, it's an adventure!"