What happened to the French is that their political leaders and incompetent generals got most of their brave young men killed. What was left reproduced, giving you the French of today.
Pity. To a lesser extent, the same thing happened to the British.
The French soldiers fought bravely in WWI, I agree that the poor quality of their generals is what got so many of them killed. America's fine military tradition is a great treasure which produces superior leadership. The French were not so lucky. My grandfather and his brother both fought in WWI for the United States, my grandfather was in the navy, his brother who was in the infantry, suffered much in that war. It was a horrible experience and mentally scarred many of the men who fought in it. I think that the French never really recovered from the war.