IMO the Schlieffen Plan was bound to fail from the very beginning, because the whole reasoning behind it was utterly false:
1) There was no way to win the war in the west before Russia mobilizes for the eastern front
In 1870/71, it took Germany more than half a year to accomplish a ceasefire, and even then France was only defeated in the sense that they gave up hope to win the war. They were far from unable to continue to cause rouble while waiting for the cavalry, if there had been any cavalry to wait for.
So long as they are any free harbours in France OR Belgium, Britain can use them to send supplies and reinforcements and ensure that Germany is NOT able to sent any troops to the eastern front.
2) Germany WAS able to fight a prolonged war of attrition on two fronts.
OK, the Germans lost in the end, but only after four years and with extensive support from Britain AND the US. Just assume no British blockade, and France fighting alone after Brest-Litowsk.