Russia, it they lost battles and whole armies, would simply fall back and trade land for time while they rebuilt.
France however, if they had lost the battle of the Marne, could have been defeated. Other than the Russians, the French committed the greatest number of troops to the allied (entente) war effort. It would depend on what terms the Germany offered I suppose to France and Britain and whether Asquith would consider it wiser to accept them or to somehow fight on.
This would in effect be the war that everyone had always imagined it would be: short, sharp, decisive and over before the leaves fell from the trees.