What would "plan B" have been? How much time would have been needed to replenish the lost troops, equipment, etc.? What would this have meant in the context of the whole war?
I don't think it's a hard rule, but /r/AskHistorians doesn't normally go in for counterfactual speculation. You'd probably have more luck at a dedicated sub like /r/HistoricalWhatIf, where this is a popular question.
This was just asked less than three weeks ago and we already answered by collecting links to even older questions of the same. https://www.reddit.com/r/AskHistorians/comments/7wo0gp/did_the_allies_have_a_plan_b_in_case_the_invasion/