Well, you can take many key battles or campaigns in history, and then say "well gee, if they didn't win there, XYZ would have happened anyways".
Example. Waterloo 1815. Ok, Napolean wins that one. Do you think he's going to reconquor Europe? highly doubtfull. He would have been brought to heel at a later date, making some other General famous rather than Wellignton and so forth and so on, but he met his end there, not somewhere else.
Germany was being attacked on two fronts, and it was defeated by a combined effort, and one of those fronts was the western front, and that front was made possible by Normandy's success. You could turn the tables and say "well the Soviet Bagration 1944 Offensive didnt matter because the Allies landed at Normandy" I suppose...but then somebody has to matter? don't they? I mean, defeat came, it didn't just appear out of nowhere, if you get my meaning.