The victors get to write history.
D-Day was a lot closer to being a defeat than is generally admitted. If Hitler had released the Panzer divisions at Calais, the Nazis might well have stopped the invasion on the beaches. Nearly all the paratroops missed their drop zones..often by miles. Some 90% of the bombs and naval artillery missed their targets by several miles. The pre-invasion bombardment was the original "shock and awe" and it didn't do a thing to the German defenses.
D-Day succeeded because the Allied command was willing to sacrifice the troops in order to gain a foothold. The correlation of forces was about 150,000 for the Allies, to 40,000 for the Germans. That's about what most military strategists suggest is the needed ratio of attackers against a well equipped and positioned defender. However, more than 75% of the Germans were positioned inland, well off the beaches...maybe 10,000 at best were on the water line. If you throw 150,000 men against 10,000, no matter how well positioned and equipped, the attackers will ultimately get through, and win the objective, as long as you are willing to accept the casualties as the necessary price for winning the battle. That's the cold, hard, calculus of war on this scale.
The first thing to go on that day was the official plan, and it was left up to these guys to improvise. And that they did.