There is one thing I can think of, though it's likely not been considered both due to its relative unlikeliness and as it's not directly tied to the Eastern front itself. By restricting themselves to a defensive action in the west, the Germans give the French and British forces a free hand to take the initiative. The French preserve their control over the mines, furnaces, and industry of the north that historically lay in lands occupied by the Germans, which was far from inconsiderable. Poking about, it looks like around 75% of French iron and coal was lost, and over 80% of their production of cast iron, steel, and coke. In the proposed counter-factual, all of this remains operating at full capacity for the French war effort even as they redouble their production efforts. Conversely, German industry just behind the border will become the target of French offensives aimed at relieving pressure on their Russian allies. If these offensive succeed at a bloody cost, devastating their Rhenish industrial base could slow the Germans down until they recover by building new industrial development far from the war front as the French historically did, and the Germans lack the access to the world markets that the French historically possessed. That may be, however, a bit of an outside shot. The terrain the French would need to break through just exacerbates the difficulties with the offensive already present in the war technology of the era, and as I'm not expert, I'm not sure if German concentration of mining and industry on and west of the Rhine quite matches the French concentration of industry along the coal and iron fields of the north or if the dominance of the Ruhr valley is overstated.
