It required the combined resources of the United States, the British Empire and the Soviet Union to defeat German in Europe.
It is true there was no way German could defeat the United States. However, without war in the Soviet Union, the probability of making in significant inroads in Western Europe until sometime after mid 1945 would have been problematic at best. Highly doubtful that there would have been a D day 1944.
The war would have been longer, but the outcome the same. When France fell in mid-1940, Germany, Italy, and Japan fought Britain alone for a year without winning.
D-Day’s importance is a myth; we already had a “foothold in Europe” a year before (Italy), and D-Day was just to reduce the losses there. As another FReeper pointed out, Rome fell before D-Day.