Remember -- this was the same army that lost to the Soviets primarily because it needed to allocate 38 divisions of German and Italian troops (600,000 in all) to occupy Yugoslavia, which had a total population of only 15 million people back then.
We will have to disagree, I'm afraid.
The Mongols conquered and ruled Russia for centuries with a great deal less power than the Germans had available to them.
Yugoslavia and Russia are a little apple and orangish. Yugoslavia - rugged heavily forested mountains. Russia - extensive plains, nowhere to hide.
While the Germans might not have been able to rule the Russians and exploit their labor, I think there is little doubt they could (and probably eventually would) have exterminated them.