It was not a slam dunk. The D-Day landing hung in the balance for that first day. If the Allies had been pushed off the beaches the outcome of the war would have been different.
It’s not clear that the Allies could have or would have tried again. Britain committed what remained of her armies. All the available material was used. It would have taken another year to rebuild and train a new force. Germany would have had more time to develop the jet, rockets, and other weapons.
The Soviet Union was pressing into Germany from the east. It is possible they would have overrun all of Europe and created a series of puppet states similar to the ones created in Eastern Europe after the war.
It is possible we would have to negotiate an armistice.
It did not happen that way. In spite of the mistakes, the casualties, and the fog of war, the landing was successful. Young boys, led by young men, got out of landing craft and advanced across contested beaches to secure enough of the French coast that they could not be thrown off. Their actions deserve to be remembered as long there is a United States of America.