You can also turn this around a bit. Had England sued for peace in 1940, then the entire might of the German and Italian armies would probably have been enough to go the extra few miles to Moscow, thus dooming the Soviet Union. But the threat of invasion held German divisions in France, and the desert campaign tied down some German and most Italian forces.
True to an extent I guess. However just what kind of impact the fall of Moscow would have had could be subject to debate. The Soviets had already evacuated and totally rebuilt most of its political/economical functions out of Germany's reach, which ultimately led to the critical reorganization of the Soviet military and the decisive counter attack. It would have had a tremendous political impact for sure, but whether that alone would be enough to signal the total downfall of the Soviet war machine is contestable IMO.
Besides, the halt in German advancement into Russia had less to do with the overall numbers concerning military resources, than the conditions surrounding the levels of preparation and strategical decision. By the looks of the first two years it seems that by on a tactical scale the "80%" was already more than enough to topple the Soviet Union into its final demise. However, in the end, the USSR survived it all, which sort of leads to the doubt that it wasn't exactly the lack of available manpower or resources that led to the German failure in conquering the USSR in under a year. In other words, my guess is whether Germany put "80%" or "100%" against the USSR, they'd never have achieved their final destination.
The Soviets would have been doomed anyway had it not been for the barbaric racial policies of the facist occupation. Stalin was none too popular, and the Germans were initially greeted at liberators. But, it became clear shortly thereafter that the Germans were starving Russian POWs if not executing them outright. Thus, the Red Army was galvinized by the knowledge that if they succeeded, there would be a chance to survive Stalin's barbarity -- if they lost, there was no chance of survival at all.
IMO that's a slightly warped picture of what was happening after the German occupation of Soviet territories. It is true that the USSR had inherent political problems in maintaining the state as a "Union" of multiple nationalities, with most of such problems coming from the very earliest days after the Revolution of 1917. However in the Russian sense, regardless of what kind of ruthless dictator Stalin proved to be to our own eyes after all these years, whether by propaganda or patriotic fervor, Stalin still remained as the savior to the USSR to the public eye in those days. In other words he was immensely popular - in the darkest hours of the USSR he was the only person on the media to address to the public in the name of 'Mother Russia' and reassure them that the country will not fail. The Soviet people answered to the call - whether by patriotism or by sheer state terror, they did.
Therefore, it is true that the German advancement did lead to a considerate number of Soviet turncoats and revolts against local Soviet occupation in the areas with highest frustration levels against the USSR, but overall most of the people of the Soviet Union were fully intent to stand against the German invaders. As much as it is silly to believe that the USSR was mobilized into action by voluntary efforts alone without any kind of state terror involved, it is also wrong to believe that state terror alone was able to hold the USSR together. In short most of these guys knew it would be a take-no-prisoners, battle-to-the-death, type of war, and they accepted it as their own fight to be fought out. Like you've mentioned, Hitler's "wipe out the degenerate Slavs" attitude did not help either.
There are multiple questions concerning just how the USSR maintained its solidarity throughout the war, but nonetheless, one thing for certain is that it wasn't going to break-up so easily, just giving up their posts and going turncoat. They weren't the French!
