If the d day landings had not happened I doubt that the Soviets would have reached the French border let alone the coast. Without the invasion all those troops that were used on the western front from the invasion to the final colapse of the reich would have been available to the eastern front. No Caen, no Cobra, no Achen , no Wahct aiem Rhine. The germans would have the resources to follow Guderian's idea of reinforceing the eastern front and stablizing it.