Apologies if this has been done before, but I wanted to go over it again with a fresh perspective. What if Helmut von Moltke the younger had stuck to the original concept of Schlieffen's plan by keeping the right wing strong. This means keeping the deployment plan the same as OTL, but as the offensive develops, instead of launching pinning attacks in to Lorraine and the teeth of the French fortress system, the German left wing sits on the defensives and lets the French offensive continue to develop? They can then also act as a reserve for the right wing to draw on and this means that the 6.5 Ersatz divisions that OTL were misused for the attack instead allow the troops besieging forts in Belgium to rejoin their units, such as von Besler's 3rd reserve corps at Antwerp, one of von Hausen's reserve corps at Namur, a brigade from the 1st army at Brussels, and the 5th corps at Etain.
Does this influence the battle at the Marne and allow for the Germans to avoid retreat? Does it also mean that the French get stuck in Alsace-Lorraine?