Well, obviously it's good that the Normandy invasion was a success, but it was inevitable that once the Allies committed to defeating Hitler, he was doomed.
The U.S. by itself could outproduce all the other armies combined, Allied and Axis. But put together with the output from the SU and Great Britain, Germany simply could not come close to matching the men and materiel of the Allies.
During the year of 1944 with Operation Bagration the Soviets totally smashed the German forces in the East Britain is often considered the poor third of the Allies, but they were ahead of the Germans in a number of key areas like intelligence and a number of other areas.
Hitler simply took on too much....even with "help" of Italy and the other eastern European countries like Romania, Hitler was doomed once he declared war on the U.S. He had no idea what he was getting into.
Incidentally, everybody should read Hanson's military books like his latest "The Second World Wars" where he lays out in detail how outmatched Germany and Japan were.