I will agree that there were huge advances in design and production between 1917 (When the design of the 1917/1919 Browning MG was set) and 1942, with the MG3. But the higher rate of fire was not an asset when YOU had to carry the ammo. I always remember the picture of the #4 in the MG42 crew, who had to carry his rifle, pack, and 10 (yes, 10) ammo cans for the MG. He was the guy who had really P.O.'d the Sgt! And, at 1,200 RPM, that was only 120 seconds of ammo!
Also, in almost every war, the guns that the enemy is using to kill your buddies seem "better" than your guns, where the effect cannot be seen.