If the French and British had held off a German advance, either in 1939 or in 1940, the equation of the war would change quite a bit. I don't think Germanm aircraft would have the range to attack Britain from the Reich, so the industries of Great Britain could continue to turn out war materiel for a force in being rather than simultaneously having to re-arm its military and churn out fighters and bombers to feed the battle over its own skies. Ditto, the submarine war becomes much harder without the bases in France - ASW patrols in the Channel and the Iceland Gap are much easier than trying to close off the Atlantic - especially if the French Navy does not get plonked at Oran but continues to operate. That again means more industrial productivity as more raw materials get through, and more value-for-money on the American-purchased arms.
An Anglo-French counterattack into Germany would not very likely have had the kind of tremendous success Hitler had in the other direction, but I could easily seethe Wehrmacht pushed back. Blitzkrieg is based on the combination of taking territory by a mighty push, then recovering strength from the resources the captured territory affords. The army poised on the borders of France, for all its might, was not equipped for the long haul, and if ther Germans are not in Paris by autumn they may well never make it. IN a war of attrition, all the advantages pass to the Allies. French and British bombers can range into Germany under fighter cover from French bases, the sea lanes remain far easier to secure, the colonies continue to supply vital raw materials without becoming a major theater of war, let alone being torn between Vichy and Gaullist parties, and I don't think a country fighting the exponents of the western world and not winning, all the while accepting the continued support of the Soviet Union, does not make a credible defender of Europe against the Red Hordes.
I'm really not sure how this would play out, though. France and Britain might be able to keep up a hard fight, but would they have the men and materiel to successfully invade Germany? I don't think Hitler would give up for anything less, and France and Britain could not well walk away from a fight they joined over Poland without getting concessions on that issue. (This gets really interesting: what about Polish territorial integrity? Note that neither ally declared war on the Soviet Union in spite of having undertaken guarantees for Polish land. Not that I blame them, but without the world-spanning conflict to follow, that question will need addressing.)
Hmmm. With Stalin's well-known attitude towards treaties, could we see a long drawn-out slogging match along the French frontier through 1940 and 41, followed by Barbarossa in reverse? Ouch.