"If 5.56 NATO were as ineffective as they say it is, then it would have been replaced by now. Don't get me wrong, I know that 5.56 NATO is no powerhouse and has failed on occasion to reliably put down a threat, but if it were as grossly ineffective as some claim it to be, then nobody would use it anymore. PERIOD.
Obviously the U.S. Military isn't shy about spending money to upgrade the equipment, so I doubt it has anything to do with the expense of adopting new ammunition, especially when all three of the aforementioned cartridges were designed with the M4A3 in mind."

This is my thinking also. My father and his buddies carried M16s in Vietnam. To a man they praised the rifle and cartridge. You would think it pretty obvious that chopping the barrel down to 14.5" with a cartridge that depends on velocity to be effective was a bad idea.