Not my intention to get into "what if" territory, but was there ever a plan B considered? What were the scenarios the British and American command were considering (the reaction of the USSR, the situation in Italy, etc.)?
Here are past answers & discussion from the FAQ section on Plans in the event of Operation Overlord's failure:
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[54 c | 10 mo] Flaired Respondents | Planning for Success, Counterfactual
[8 c | 1 yr] Counterfactual
Ask followup questions by commenting with /u/ appended to a flaired specialist's username i.e. /u/OldWorldGlory.