Gee, Calvin, those "underequipped,hopelessly outnumbered" Germans seem to have put up a pretty good fight. Or was Eisenhowers fears in the first days of the invasion just a scam? There are a lot of D-Day vets that would argue the point regarding the "real effect" of D-Day.
They put up a hell of a fight. I'm currently in the middle of reading the story of the 12-SS-Panzer division, which was a group of 17-18 year old kids thrown into Caen in the first days of the invasion. Facing total enemy air supremacy, and outnumbered 4 or 5 to 1, they kept Monty and the Canadians bottled up for months.
A big problem seems to have been Monty's inability to understand modern armored warfare. The author, who was the Divisional 1A at the time, noted that Monty used the tanks to support infantry assaults, where a German general would have broken out quickly by letting the armor have its head and using infantry to mop up in its wake.
Of course, Patton later corrected this failure of imagination, but by that time the front-line Panzer divisions were at 40-50% effectiveness, with no remaining heavy weapons to speak of.