Eisenhower was indeed worried about the failure of the landing and had this backup speech ready in case of such a disaster:

Our landings in the Cherbourg-Havre area have failed to gain a satisfactory foothold and I have withdrawn the troops. My decision to attack at this time and place was based on the best information available. The troops, the air, and the Navy did all that bravery and devotion to duty could do. If any blame or fault attaches to the attempt it is mine alone.

But I want to ask clarification to your question. There were multiple beaches along the 37 mile landing zone. Are you asking what would have happened to the successful footholds or are you asking if every beach landing was a failure what would have happened?

Because if every landing was a failure the men would have been slaughtered and the lucky ones taken as prisoners of war. Because that is the only scenario where I would call Overlord a failure. Say if Utah was a massacre, there was still Juno, Omaha, Juno, Sword, and Pointe du Hoc.

In the unlikely possibility that every landing was a failure, still remember how late in the war this was. Russia was giving the Reich a pounding on the eastern front. Ultimately another landing would have been tried as the red tide pushed closer to Berlin.