I got out of the Army in 2008. At that time, I was the Battalion Master Gunner on the tank side of the house, also spent a couple of years as a unit armorer. We began fielding the M4s in 1999 to the majority of the Tank units as well as some of the infantry units. Pretty much all SF units already had them. The units that didn't get M4s began to transition to M16A4s (also some full auto M16A3s to certain units) a few years later. When I got out, all of the combat arms units that I know of had m4s, some SF units had some full auto M4A1s. When we deployed to Iraq, we had several soldiers that were Designated Marksmen and used M14s.
During this time period, several rifles had been tested, but not approved. The XM29 had the 5.56 and 20mm grenade launcher in one package, but it had issues. The XM8 had been tested as well. It was the 5.56 portion of the XM29. The SCAR and HK 416 have also been looked at. I think that the military as a whole just has issues with scrapping all of their current, proven, inventory of weapons. I am sure that 6 or 7 hundred thousand select fire/full auto SCARs for the Army would carry a heafty price tag.
Personally, I believe that a 62 grain 5.56mm round is sufficient in the hands of a well trained soldier. The problem is that more and more soldiers that are not in combat arms units are ending up in the line of fire and they have not had the amount of training that the traditional combat units have had.
Will the military change weapons, eventually. It is a matter of politics more than a matter of having a weapon that is out of date. If the M16/M4 platform was out of date, we wouldn't all be buying/building them.