The French had blocked the thrust through the Ardennes in 1940

This would probably also lead to changes in another battle, the winter war. It is considered highly likely that with the 1938 killed officers the Red Army could have won the Winter War, and then Hitler would not make his assumptions on the Red Army´s condition like he did in 1940 and decided to start Barbarossa.

 

I don't think the French could have stopped the German army at this point. The German army had gained some battle experience with the war in Spain and Poland. The French were still in the mindset of WWI. Although they could have made things more difficult, the German army would not sit down to trench warfare. They would simply use their speed and tactical maneuvering to eventually surround and cut off the French as they had done with the Russians later. The French were to slow to adapt to the times.