The weapon and caliber used in Vietnam was effective. The political leadership bending over backwards to spoiled Baby Boomer brats who were being manipulated by opposing interests had a lot to do with the lack of leadership and willingness to win.
In WWII, if it took carpet bombing inner city areas with napalm and wreaking massive destruction on the German people, it was done. Hanoi? Ohh, noo, we can't do that.
Was it a war or not?
Blaming the resulting loss on one weapon used in the field is myopic and ignores the other 90% of the battle that went on. Our troops did fine with the M16, our government, not so well with gaining important concessions or even approaching the situation with a informed view - as they have admitted.
And as said, the 7.62 wasn't removed from the battlefield, it was still being humped over the paddies and trails as an MG team with gunner and assistant gunner, set up in perimeter bunkers, and hung from pintle mounts on APC's, tanks, and helicopters. It was still there where it counted, given that it delivers when you can see past 500 meters and have an enemy exposed.
It does no better in thick bush that stops any bullet in a few hundred yards, and no one can see past 50. In abusive heat and humidity, carrying a 12 pound rifle and half the ammo isn't a major advantage - a light weight rifle and twice the ammo does exactly as the German General Staff predicted. It puts more rounds in the air, and gets more hits.
Those of us who have carried both over the years can understand, those who repeat an old familiar complaint with no knowledge seem rather uninformed about it all, but that's what the internet is about.