The Critic Said:

The 'What if the US Never Went Into The War' question had been debated 
for a long time, but whenever it comes up, most historians agree that it 
would have ultimately made not much a difference.

The US's entry into the war was indeed a smaller contribution than 
Britain's and Russia's.  However, that does not justify your statement
that its totally implausable.  In fact, there has also been a study on
exactly what the Germans did to lose WW II, and the big points were...

- If Hitler had waited 1 year, the German Navy would have finished its 
  building program.  This would have allowed a stronger strangle hold on
  Britain.  It would have also given them more confidence in attempting
  for an amphibious assault.
- If Hitler had ignore Goring and had gone for an amphibious assault of
  Britain instead of trying to bomb it into submission, right after 
  Dunkirk, Britain would have fallen.  They were not heavily mobilized,
  and the troops that had fled Dunkirk left all their equiptment behind.
- If Britain fell early in the war, reistence in the colonies would have
  diminished, especially their ability to reinforce and resupply.  The 
  Afrika Corp would have finished its drive into the Middle East.  
  Operation Barbarossa would have included a strike from the Middle East 
  into the Caucasus.  Russia would have lost the vital oil fields early 
  in the campaign, and would have been unable to replenish its reserves.
  This would seriously hamper their new weapons, the T-34s and Yak 1/3/9's.
- Japan would not have been mauled in their attacks on India if Britain
  fell early.  Russia is a non-factor as Japan never declared war on them
  and the USSR only declared war AFTER the atom bomb fell.
- Without Britain to fear in the west, a hecka lot of divisions would have
  been freed up from guarding France.  Many of these were top line units
  which would have aided Operation Barbarossa.
- If Hitler had not been blinded by pride and racism, he would have known
  that Operation Barbarossa would not be a knock out blow.  He would have
  planned and prepared for a long campaign, not the half-year one that 
  he had planned on.  They only prepared winter clothes for a garrison
  force, a factor that hurt them much during the Russian winter.
- With an early posession of the Caucasus oil fields, Hitler would not
  have diverted troops from the Moscow front down to Army Group South.
  Moscow's value is not only political, but material also.  It had great
  stores of food and oil, but it also was the central point for all
  railroads in western Russia.  The effort to supply Leningrad would have
  shut down, and the siege would have ended either in the starvation of
  Leningrad, or in the Russians attempting a breakout which would have 
  been a bloodbath.  The defeat of Leningrad would have also freed up
  Army Group North.
- If Hitler had listened to his Generals and allowed them to pull back to
  more defensible positions after the German offensive ground down in the
  winter of 1942, they would not have gotten so beat up during the Russian
  Winter Offensive.  In turn the German Spring Offensive would not have 
  been conducted by tired and beaten up troops.
- If Hitler's engineers and scientists had been allowed to conduct research
  as usual instead of being pushed into making a super-weapon (V2,
Ferdinand,
  Maus, etc.) they may have kept pace with the allies in terms of weapon
  design.  In fact, the Panzer was more than a match for the T-34, and the
  Bf-109 and Fw-190, though outclassed by the P-51s and Yak-3s, were
piloted
  by the greatest fighter aces that man has seen.  If so much genius and
  work had not been wasted on Hitler's "toys", they would have created 
  machines that let them match or even out perform their opponents.
- Finally, Hitler had been complacent, and had not ordered Full War 
  Production until they were pretty much defeated on the Eastern Front.
  Even late into 1943, they were at pre-war production levels in war
  materials.  This was done to keep the people happy, but was a serious
  blow against the war effort.

Your argument that the Allies exceeded Germany in production and man
power is absolutely true.  But that does not naturally point to victory.
If that were true, the US would have lost the early war in the Pacific,
when they were absolutely outnumbered.  Through the leadership of skilled
commanders and a bit of luck, the US Navy did not get wiped out.

As it has been often said, the Third Reich would have triumphed
save for Hitler, but it would not have existed but for him.