There was a problem with the Garand family of weapons: round output in the early days of the Chinese offensives. The Chinese were, strangely enough, more plentifully equipped with more men with more varieties of automatic weapons. It was thought that American soldiers needed to be given lighter rounds and higher volume of output. Thus, Eugene Stoner's Armalite found favor within the Army that had a baleful experience in Korea.
However, in Afghanistan it was found that lighter rounds (from say, the 9mm family of Baretta weapons) didn't necessarily put the Jihadi to sleep. So there has been a trend back to the 1911 as a sidearm. I haven't heard about a trend away from the M-4 as it is the Government Issue rifle, but it does appear that people are looking for a happy medium between round output and stopping power.
So there has been a trend back to the 1911 as a sidearm. ............... Like something was wrong with it? Can never reason why they dropped it other than to up date it to a newer look. If they wanted 9mm they could have modified the 1911. If you go house to house, why mess with a M16 derivative when you can unpack, degrease and reissue the stored Thompsons. (Rhet)
To build on what you said, there is a fundamental problem with the assessment that lighter calibers create greater strain on the enemy by creating wounded vice dead. The problem is that we studied ourselves, but we don’t fight ourselves. Our enemies are dirt bags by nature and aren’t overly worried about their wounded.
Additionally, the Jihadists want to die, wounding them only keeps them alive to detonate.
Finally, we used to have different weapons for different tasks. In room to room fighting in the third world, you need a round that is capable of penetrating cinder block walls and killing a man on the far side, yet it only needs to be able to do this from close range, not 200 yards.