The Atlantic Wall would have failed at its task, even if it had been fully completed. Allied air and naval superiority could paralyse it, the Allies had plenty of experience in fighting through similar defences, and it was useless after a breakthrough. The Allies just had to win enough of a foothold to pour men into, and they were easily capable of doing this.

The British and Americans would always have naval superiority over the Kriegsmarine, even if Plan Z had been fully completed. Both the USN and the RN were larger, and both had far superior training. Similar arguments explain why the Luftwaffe would be unable to win air superiority - Germany was outproduced something like 4-1 in planes by the US alone.The inability to win naval superiority meant that the Atlantic Wall had to be manned from Narvik to the Pyrenees, as troops could be landed anywhere on that coast. In addition, troops to counter-attack landings had to be held far from the front, so that they could react to landings anywhere. In June 1944, most of the Panzer divisions in France were held far inland, instead of close to possible landing sites. This would be true in this theorised later attack. Allied air superiority could prevent these troops moving to counter-attack, by destroying rail lines and similar transport links. Paratroops could be dropped, as they were on 6th June 1944, to cover the flanks of the landing and to hold the routes the counter-attackers must use. If they reached naval gun range of the front, then the Battle of Gela, during the invasion of Sicily, is an instructive example. During this battle, a German attack on the beachhead was destroyed by naval gunfire called in by the American defenders. Allied naval superiority meant that overwhelming firepower could be brought to bear anywhere they chose to attack.

The Allies, especially the Americans, had plenty of expertise with highly opposed amphibious assaults. Battles such as Tarawa and Kwajalein had taught the Americans many lessons about how to deal with fortified positions - the development of amphtracks, better communication between ship-shore, better assault infantry tactics were all results of these battles. As 100% completion of the Atlantic Wall would require the assault on France to be delayed until at least 1945, the Americans would also have learnt the lessons of the Marianas, Iwo Jima and Okinawa. The British had less experience, but, as their production of the engineering tanks of the 79th Armoured Division (Hobart's Funnies) shows, they worked well with what they had. Any Allied attack on a fully completed Atlantic Wall would have resembled any number of the amphibious assaults of the Pacific. The first wave would have taken heavy casualties, but ultimately the Allies would be ashore in force.

Finally, the Atlantic Wall was too thin. It was a thin, but well defended coastal crust, with very little behind it until the Rhine was reached. Had the Allies broken through the Wall in one place, they would be well positioned to roll it up from the sides and rear, where there was little protection. Troops could not be pulled off the coast to meet this threat, as to do so would expose their vacated positions to attack, due to the Allied naval superiority. Preventing a breakthrough could be done if a counter-attack could be effected, but as discussed above, this was unlikely in the teeth of Allied air and naval firepower. The coastal crust also ensured that the majority of the defence's strength was within easy reach of the Allied fleet, with its massive firepower. A better defence would have resembled the British coastal defences, or the Japanese island defences seen in the Marianas and Palau campaign, or on Iwo Jima and Okinawa. These had a thin delaying line on the beaches, and the major line of strength inland, with other delaying defence lines inbetween. This allowed the defender's main forces to remain out of the line of fire, while attritting the attacking forces. The defenders could not be outflanked by further landings, as they shortened their lines by retreating inland.