I've always wondered why the French considered Brittany for a national redoubt but not Normandy. Suppose they retreat to Normandy? They'd have an excellent port in Cherbourg. They'd be closer to air cover based in southern England. Shipping from southern England would have a very quick transit. A defensive line could be set up from Caen through hedgerow country and then to Avranches on the other coast. The terrain really favors the defense. In addition, naval gun support would be available from a combined British/French fleet. The Allies could set up radar stations in Normandy and tear up the German air force. Luftwaffe exhausting itself in 1940 all sorts of possibilities open up. Likely there never is fighting in Greece and North Africa remains French. Germany could bring many divisions to bear but in so few miles of front they'd be unable to maneuver effectively.