This is one of my favorite What-ifs, and I game it every chance I get. I have played two WW2 by e-mail games in which France almost pulled it out.
In the first, the Germans attacked as historically, but the breakthrough was narrow. On one side was a powerful French force, on the other, the entire Belgian army and the BEF. Orders were given to pinch the gap shut.
And the next turn (June 1940), the BEF bugged out. They left in full retreat, throwing the Belgian lines into disarray, clogging the harbors at Antwerp, and assuring the victory of the Germans. Grr.
In the second game, the French attacked into the Augsburg area and gained some ground before stalemating for the winter. In the Spring, the French thrust forth and took Cologne. Orders were given for a knockout punch past Saarbrucken, but the Germans had regrouped, and they beat the French to the punch by six days.
*sigh*
I blame it on the bias of the German GameMaster.
In no way do I think the Fall of France is inevitable. So let's wave our hands. It's 1941, and the Germans have been unable to bring about the capitulation of the Allies. What happens next?